mcintosh c2600 and ma6600 question

Wasn’t good. Works with regular amp much better. I sold ma6600

Low end totally collapsed, upper bass became muddy, soundstage narrowed and overall sweetness disappeared. 
When I connected to to MC452 there was a noticeable improvement in all areas and markedly improved from ma6600 when it was used as an integrated 

I do regret selling the ma6600 though 

separates definitely sound better, but I wish I kept the ma6600 for my other system, it was a sweet sounding amp

Depends on your speakers. If you’re going the tube route then why not go fully tube and get a tube preamp as well. I’d go for the new C70 or C2600. A C22 would be a nice match to mc275, buy I think they discontinued the line. Whats does the rest of your system look like? 

I sold my ma6600 for 3k here. If you’re buying brand new Mcintosh from a dealer you should get at least 15% off MSRP, at absolute minimum.
If I was in your situation, I’d go for an all tube system and a preamp without a dac (c22, c70)
you won’t regret it.
Keep in mind that if you get the aforementioned Mcintosh system, you’ll end up with amplification that would outmatch your speakers by a long shot. You’ll eventually want to upgrade your speakers to BW803D3 or similar to let your amps really shine. Ideally you should optimize your speakers prior to upgrading your amp/pre. This was the mistake I made, I had a 15k system driving 2.5k speakers. When I finally got my SF
Elipsa se, I was able to get the most out of my amps. I found that ideal ratio of money spent on amp to speakers should be 1:4. Thus, for your speakers I'd just get a integrated amp for now until you're ready to upgrade your speakers.