In the meantime the cables have arrived, improving the sound enormously
. I had set up the subs with some cables that I still had, but these were short, so the subs were not only located close to the amplifier, but also quite close to each other. With the new longer cables I could locate the subs as per Earl Geddes recommendation: the biggest sub in the front corner, a second medium sized sub at the back, and the smallest sub along a side wall. This is in a large room, so room modes were no longer bunched together. The resulting sound was very much cleaner, with significantly better defined bass. I must admit I was surprised by the impact of this theoreticaly better placement. The next step will be to equalize them with MSO, but that will take a bit more time.