How many subs?

I found whatever I did, multiple subs interfered with the sound from the main speakers.

@bilateral In that case they weren't set up correctly. The most common problem is crossing them over too high!

Here's a  tip for your sub. Point it at an angle towards a wall; imagine bouncing the bass wave off of the wall in a manner similar to a cue ball in pool. This will allow the bass to be more able to not form standing waves and might sort the room out nicely.

the drive rack was not used for purpose, it was what it could do.  I wanted a balanced product that could give delays up to 10 ms (not including latency).  When the Acoustic Engineer said 3 ms delay there and 9 ms I got someone else to program it because we could not work it out.

@bilateral unless your room is enormous you don't need any delay at all unless your subs are active at higher frequencies. But most speakers will go well below 80Hz; that's sort of the 'magic number' in most rooms; if the subs do not put out information above 80Hz they won't attract attention to themselves. Since all the bass at that frequency and below is reverberant its easy to get them to blend.

@bilateral No need for any delay in such a small room! If your 'acoustic engineer' thinks otherwise, fire him- he's doing you no favors.

Here are some facts to consider: at 80Hz, the waveform is 14 feet long. It takes a full cycle to pass the ear before the ear can tell its there; it takes a few more to tell what the note actually is.

By this time the bass has bounced all over your room. This means the bass is 100% reverberant.

For that reason you can use a mono bass signal in most rooms except those that are very large. The subs need not be anywhere near the main speakers as long as their output is kept below 80Hz so as not to attract attention to themselves.

The problem you run into in a room like yours is something called a 'standing wave', which can cause bass to cancel. Cancellation can't be corrected by bass traps or digital room correction since that would simply ask for more amplifier power which would get cancelled. To break up the standing wave you need multiple subs which are asymmetrically placed about the room. This isn't the sort of thing that 'takes hours'.

For this you need 4 subs to be the most effective. This technique is called a 'Distributed Bass Array'. It will breakup any standing waves and will take care of 95% of the room's bass problems. Only after they are installed should you try bass traps and/or room correction, which can do the other 5%.

End of 'facts to be considered'; now for the anecdotes:

My speakers at home are flat to 20Hz. I had a significant standing wave. So I needed two subs to break it up. One is to the left of my listening position and the other is behind me and to the right. This worked a treat and literally took a few minutes to set up.

Audiokinesis makes a sub called the Swarm for this purpose. Its designed to be set as close to the wall as you can get it. In my room, I have the drivers actually facing the wall so as to be certain its inside the room boundary effect. By so doing, the Swarm subs are also flat to 20Hz. A single, inexpensive subwoofer amplifier recommended by Audiokinesis drives the two subs.