Use Marantz 6006 as pre amp to a Rotel 1095?

definitely worth trying--the amp section is the achilles heel on most mass market avrs. i'd be surprised if you didn't see significant information with the rotel.

I used the Rotel 1095 in the same manner with a Pioneer receiver with good results. It clearly improved the sound into 4 ohm Thiel speakers.

As always, YMMV. Years back, I paired a Rotel 1066 amp (150 watts x 3 in bridged mode) with a Pioneer AVR. The improvement was not very big. Until I upgraded to a better pre-pro, I didn't realize that, in that Pioneer AVR (a 24TX from around 2000), the amp section sounded pretty good; it was the pre-pro section that was really bad. I ended up using that Pioneer for surround amplification only for a few more years.

FYI, newer Pioneer AVRs sound much, much better than they did 20 years ago. I now have a newer one which I use for surround processing and amplification of the surround channels in my combo HT/2-channel system.

So, my point is, by all means, try a new power amp. But if you're not impressed by the increase in sonic quality, consider a new pre-pro or AVR.