OK, some good advice here. All rooms need treatment, it is paramount for good sound. EQ/DSP may help after getting the room sorted out. The problem with room correction electronics is they can’t reduce the overly long decay of sound in an untreated room, how can they? They also can do nothing about the full nulls created when low frequencies recombine out of phase. It does not matter how much power you pump into the speakers the sound will just cancel with the same power. Partial nulls can be boosted but if you try and correct for example a 15dB dip you will not have enough power. It requires a doubling of power to achieve a 3dB boost. Try doubling your amp’s power 5 times!
That’s what Krell is for: you too can break your speakers.
When sound is left to decay naturally in a small space (the average room) detail is obscured by the confused and smeared sound. So determine the required time it takes for your sound to decay by 60dB across the full spectrum, known as RT60 or T60. The average room needs T60 to be about 300 to 400ms.
RT60 (and T60) is somewhat ambiguously named. The 60 dB decay thing is the original Sabine equation. T60/T30/T20 are methods of measuring the decay time (they all return the same result, the lower numbers just extrapolate from a smaller drop in level).
Typically mixing rooms have a lower decay envelope and preference for a listening room will depend on music preference. I like electronic and studio-assembled stuff so I’ve no use for added warmth or envelopment from longer decay times (which benefit classical, for example) so my room is closer to a mix room, around 200 ms. Regardless of preference you also want consistent decay across the frequency spectrum (there’ll be an increase in the lowest octaves).
The thing to do is download a great free program REW and measure your room. You will need to buy an inexpensive mic. for the job. This can provide a CSD plot of your room showing the peaks and nulls. Yes, you will need bass traps. REW can help you best position your speakers and show the effect subs have on the performance.
Smaller rooms (most people’s listening rooms) complicate things a bit as decay to a uniform diffuse field doesn’t happen and RT60 isn’t super-accurate, so listen and judge accordingly. REW has a new method for smaller rooms, so try that.
Multi -subs can and should be used to smooth out the bass response. The optimum would be to treat the room in combination with 2 or 3 subs and you will get to hear, probably for the first time, the music info that has been ’black-holed’ by the nulls. Some who have heard of multi-subs envisage thunderous bass but properly placed and dialed in they enhance the entire spectrum from top to bottom. Think of them as tuning units.
WARNING: place a cushion between your feet in case your Jaw hits the floor 😁
Careful positioning of full-range stereo speakers to minimise nulls followed by judicious EQ to tame peaks at the listening position will often work (or it does for me, in an open timber house but will depend on your room characteristics, so quite different in a concrete apartment block) and yes the difference is dramatic. I don’t need bass traps specifically (approximation of reverb time rises to ~400 ms in the 31 Hz band which is still fine) but I need to reduce a 15 dB peak at 50 Hz (from the room’s second longitudinal mode) at the listening position. Subs give bass extension for limited-range speakers of course, and two or more subs can be located to give more even bass response over a wider listening area. Setup is more complex in that case.
One of the best articles describing small room acoustics in reasonably simple terms is this one, covering decay and the different frequency ranges (specular, transition and modal) to consider when setting up gear and a room.