McIntosh SS 'sauce' is to do nothing to the signal, that was my experience.
I used a McIntosh SS Pair for years. Amp: MC2250; preamp: C28.
Decided to go back to tubes, sold both on eBay,
AMP
I chose a used Cayin Tube amp, A88T, 45 wpc, plenty for my efficient horn speakers. Just got lucky, they are a terrific combination. Designed in Germany, to sound like McIntosh Tube Amp, built in China. Terrific quality, awesome sound.
This convinced me about the Cayin
http://www.6moons.com/audioreviews/cayin/a88t.html
If you consider tubes, perhaps look for external bias meters and external adjusters.
PREAMP
I question your retaining the SS C27. To me, if I kept my C28, that would be keeping 'do nothing'. I am very glad I went back to tubes,
I sold my SS C28 and bought a very vintage McIntosh mx110z Tube Tuner/Preamp. Love it, and it's phono is the best I ever had. I was not fond of the SS C28's phono. I use a SUT so I can continue using mx110z's phono. It has 2 phono inputs, and trim controls to match different cartridge signal strengths, and a 3rd input with trim control. Your C27 probably has that also.
The mx110z tuner is awesome, capability and sound quality. I only listen to one station: Jazz, 88.3 in NJ, but it pulls in plenty. Again, best sounding FM I ever had. I'm 72, I've had a few. They say it needs a good signal, I have a large dedicated FM antenna in the attic right above.
This convinced me about the mx110z
https://positive-feedback.com/Issue77/vintage_mcintosh_experience.htm................................
OR: Keep your McIntosh SS Amp, change only your preamp.
This Vintage Luxman Tube Preamp tempts me greatly even though I don't need it
https://www.ebay.com/itm/Vintage-Luxman-CL35-MKIII-Tube-Preamp/303871182981.............................
