There are many to get the flattest possible bass response in your listening room. One is not to have a room, of course. Cos walls reflect sound waves and these then build on each other to reinforce themselves leading to several one-note bass issues. Several? Yes, because the walls on your left and right have one "standing wave", one between your front and back walls, and another one between the floor and ceiling (which is why sloping ceilings are better from a hi-fi perspective). And there can be multiple smaller ones too.
The other way to get a flat bass response is to install bass traps. These slow down the bass passing through them and bouncing back (usually off the corners), thus reducing the chance of a wave of sound building up. If you can, do so. Bass traps are great.
The last way is to utilize each of these waves to help even out the bass response in the room. Every standing wave also has a dip, not just a peak. So what if the second standing wave has it's peak where the other standing wave is a dip?
That's how multiple subs help.
They even out these peaks and dips. It also means they need to be in different locations in a room. Both in corners will accentuate the problem. The second needs to be in a different position so it's peaks and dips are not adding to the problem!
I suspect a third or even a fourth sub will help smooth the bass out even more.
My own system has 2 powered subs. One in an alcove together with the main speakers (yes not the best place for loudspeakers, never mind a sub!). And the other in almost the centre of the room under a sofa. They have certainly evened out the bass in the room. You can walk the entire length and breadth of my open plan house and not have spots where the bass is crazy loud, or missing. Something which is always the case in a listening area with a single or no subs. Plus small subs have a better transient control, ie. tighter bass.
Give multiple (small) subs scattered throughout a room a try. You'll be glad you did.
In the end Hi-Fi is not about systems, it's about the music. But we've got to get the system "right" first. And Audyssey room/system calibration is one of the best 'fixes' available.
I'm very blessed in that I stumbled across a solution to Audyssey's issues with audio files.
Cos Audyssey often gets a bum rap from Hi-Fi 'experts'.
You've got to understand that Audyssey is SMPTE-standards based. That is, movies. Blu-ray and DVDs.
For audio, it's another matter entirely.
Movies are set to output at a certain loudness. Then loud parts are LOUD, and soft parts are soft. Some people hate this, and for that you can reduce the dynamic range from Full to Half or even less. So that explosion in your action movie does not awake the neighbourhood at 2:00 AM when you're feeding your insomnia. But when you ARE watching a movie and want the the full effect, you can have it.
To do so, check your DVD or Blu-ray player settings.
If you have Audyssey it's right there too, under "Dynamic Volume". At least that's what it's called on Denon AVRs. Check your user manual carefully. This 'feature' is available on all flavors of Audyssey. You can choose from None, Low, Medium and High(ly reduced range) options. For Hi-Fi it should be set to "None" of course, but when I have dinner parties background music benefits from a "High" reduction between a song's loudest and softest bits.
The issues many complain about start with Audyssey's "Dynamic Loudness".
This is another Audyssey feature that realises you've lowered the volume of your playback and changes the sound to compensate since at lower volumes our hearing is less sensitive to bass and - to a less extent - treble sounds. So, bass (mainly) seems to disappear and loose it's presence when we play things below their intended levels. So Audyssey is clever, automatically increasing your bass, yes? Well, not quite.
This sort of auto-adjustment is dependent on the levels recorded within the source material. Since Blu-ray and DVDs are SMPTE calibrated, they work well with Dynamic Loudness's automatic adjustments.
But such smarts are Hell with most music - Internet radio, digital downloads, YouTube files and CDs.
This is because of The Loudness Wars. Even though the capable dynamic range of the lowest forms of digital audio is 40dB more than analogue systems, music producers insist on making their digital files and CDs louder than everyone else's. So in the main digital music input into Audyssey is some 10dB to 15dB higher than your standard SMPTE digital video, throwing Audyssey off it's carefully calibrated settings.
The easiest way to get around this is to set your Audyssey "Reference Level Offset" from it's default of 0dB to -10dB, or even -15dB. Remember to put it back to 0dB when you play your movie tho!
But with audio, how to tell which is right, -10dB, -15db, or -5dB? There is a way.
But first, how did I discover this? It happened purely by accident. I had a DVD of pop hits that sounded fine but the same hits off CD were awfully bassy. That's because when I dropped the volume control on my AVR from -10dB to -25dB (from reference maximum) Audyssey added more bass to compensate - since I was playing my "movie" at a much lower level than designed. How was Audyssey to know my Blu-ray player was playing a louder than usual audio CD, and not a SMPTE-calibrated movie?
Once I figured that out my music never sounded better!
So which, -15dB, -10dB or -5dB against "Reference Level"?
I took a different approach.
I re-calibrated all my digital audio files to SMPTE levels. That way no more changing Audyssey settings for when I played movies vs music.
Using MediaMonkey as my audio player software I went to Tools, Analyse Volume and allowed MediaMonkey to check the RMS levels of my tracks and store metadata on how much to change it by. The default setting is 89dB, but over the years I've realised 85dB is the right settings. Going forwards your tracks are now "SMPTE calibrated" and Audyssey does it's magic perfectly with these tracks whether played softly or LOUD.
The other benefit of this happened recently. Realizing that Spotify has 320Kbps quality audio I signed up and started streaming. The default level was too loud (of course!). The solution was 'simple'. I played that same track off my private collection and also off Spotify as close in sync as I could. I then adjusted Spotify's level till it matched MediaMonkey's. Now even my Spotify is "SMPTE calibrated" - correction, properly Audyssey-calibrated!
Don't give up on Audyssey. There is no better-value room and speaker/sub calibration system out there! If you haven't got it, get it (preferably with 2 non-mirrored powered subs)!
Note that sub/mains calibration is a critical thing for great hi-fi. Slow, ponderous, heavy subs are no match for main speakers with super light highly-transient capable drivers. Audyssey allows them to match up so both move together at the same time. For example, when a bass drum is kicked (sound isn't a single frequency, it is a spread of frequencies) this affects your sub, woofer and even your tweeter at the same time. So any sub-only "tuning" system is thus inherently flawed - just fixing EQ isn't enough. Since Audyssey controls both the sub and mains, it manages this elegantly and perfectly. As for systems with a sub and no DSP, go figure. No wonder most audiophiles who add a sub quickly retire it.
Enjoy your music!