Need a new amplifier. Need a good match for my Audio Research LS7 preamp

I agree with yogiboy that brightness is a common comment about that speaker.

Whatever you do, do an audition if at all possible! Its very easy to flush lots of $$$$ down the loo otherwise....

I would use tubes, but because of the design of the speaker you will need a tube amp that employs a bit of feedback. This will sound smoother than most transistor amps you can find. The RM-100 by Roger Modjeski would be a good amp if you can find one, I think they are discontinued. The Cary SLM-100 or similar would be another good used choice.

Tube power is more expensive than solid state, so to get the same power I think used is the way to go. Good Luck!

How's your room acoustics? Brightness at higher volume may indicate a "live" room. Even the cheap acoustic foam is better than nothing, but thicker the better, like 3 or 4 inch. I covered Roxul panels to good effect.
I agree with the amp suggestion to the extent that it won't be as shrill as the Arcam, but the better amp won't tame a shrill  speaker and room.

agree re rom tmts.

also, maybe not any FET type but J-FET 

it is getting old and tough to find,  but a Muse One Hundred Sixty sounds like a perfect fit. 

A lively room is definitely something to be considered! The room is at least half the sound of any stereo so proper room treatment is important.