I may be wrong, but taking into account the rated power, I suspect Eloquence Series amplifiers use a class D output topology. If I'm right, you'll have a downgrade in terms of overall refinement.
You'll trade off mid and high frequency refinement by sheer power, dynamics and transient attack. A well desgined class A amplifier, like the referred Sugden, definetely sounds more liquid and natural. I reckon you won't like the quality of trebles of the 150i because of lack of harmonics, a problem that is intrinsic to the technology of switching amplifiers. It affects the full frequency (bass sound cold) range but is mostly apparent on the top end.
If I were you, I'd consider to get a class A Sugden power to use together with your IA-4 integrated amplifier. Other way to go is the adoption a well designed class AB integrated amplifier, like Luxman L-509 (bipolar output) or Accuphase E-460 (Mosfet output).
Hi Audio Maniac,
You're absolutely right and that's why I discounted the Perreaux.
Your other suggestions make sense, all be it a bit out of my price range, but I'm trying a MF AMS35I at the end of next week... whether or not it sounds better the than IA-4 remains to be heard.
Cheers
Mac