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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.
Airhead-
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
Sadly I'll be shown the door before the glass cupboard is. It was made for a piece of woodwork, family heirloom from my GFs grandparents.
Your advice, as always, is really appreciated. At the moment I have less bass than i expect in quite a specific range, probably around -3db in the 100-150Hz range if my ears are functioning. The drop was wider before the new amp (perhaps -6db at 100-300Hz) but it's still definitely there. I don't hear much in the way of bass boom in listening position , though I haven't bothered putting my head in the corners of the room. More importantly, if I move around the room I don't hear any meaningful change in the sound profile. This tells me it's less about the room mode and more to do with how the air is being driven, though I totally accept I'm making things harder for myself by having the speakers so close to the wall along the long edge of the room.
There's an alternative perspective on this which is the highs and upper mids are just a little too present due to the perky little cyrus amp, and I could treat this with HF sound absorbing panels, but I'm sceptical because even at higher volumes the speakers aren't delivering - the weight in the upper and mid bass range is just a little bit disengaged. And this is why I was looking at other amps yesterday, I was worried that it's the combination of amp and speakers that is the problem. I know I'm being picky but I have decent hearing and I know it's just not quite right yet.
Tomorrow I'm going to walk around with an (albeit uncalibrated) mic and run a pink noise generator to give me a better sense of what is going on in different areas on the room.
Edit: the other thing I'm going to do is temporarily move the speakers to the short wall, place them away from the wall and sit myself down one third of the room's length away from the opposite wall. If I get the sound I'm looking for I know it's at least possible from the set up without spending any money on further equipment (which is my goal).
Some good news, GF hasn't noticed the extra black box on the shelf. Win!
cheers all again