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I have the possibility to move to mono amps - the cyrus power amps will comfortably run in mono and another will only set me back £300. Speaker cables are 3m. I could easily hack off a metre if necessary.
You've given me an idea though. I can try a different source to the power amp...that didn't shed any light :/
freddo-
I thought that moving to mono blocks would be easy enough from where you are at now. Changing that was a big step forward for me. Mono block amps closer to the speakers, 2ft speaker cables and no pre-amp. I did end up increasing amp power from 60 - 250w as well, that sometimes helps with woofer control.
Then I started to hear what the speaker position was doing. I've still made compromises in terms of speaker position and I'm now running a digital desk which functions as a pre amp but it all still measures pretty flat at the listening position.
I have 2 pairs of speakers I'm swapping between so I can see pretty quickly what is room and what is speaker. Sometimes that helps, mostly it just doubles the amount of work required!
Airhead
The only other thing that stood out to me was the length of your speaker cables. I've found through experience that short speaker cable runs from mono block amps can improve bass control. Certainly moving to mono amps (and no pre amp) made a huge difference on one of my setups, especially in the area you are describing as weak. It is common to see huge resonant dips around the 100-300hz area, and the universal 50hz bass bump of course which makes any mid bass loss seem much more severe.
My only concern about your current setup is even if you fix the frequency issues will you be able to get a decent soundstage at the listening positioning. One of my favourite forums is hosted by a mastering engineer, he made a very good case that systems should be judged on how well they reproduce reverb in the upper mids.