Audio Research amp for an ARC LS-7 preamp

Any of them ( ARC amps) will be fine , but really matters what speakers you are driving and how large your listening space is..

I had a an ARC VT60 SE when I had one, that was the amp that was marketed with the LS7 Its is a great pair. The VT60 is 50 watts and best for a small-med size room. I added a powered sub and it was an awesome little system.
You could step up to the VT100 series if you need more power. These combos are close to the best sound you can get
Make sure to have an awesome quality source and your all set.

Thanks for your responses....the ARC setup will be my next one, in a 14' x 24' room with one of an assortment of DIY speakers I'm building (Tony Gee designs, mostly). Currently I'm in a smaller room with a Raysonic tubed integrated amp and CD player, and Celestion SL6s speakers.