New McIntosh amp build quality

In terms of the MC 275 VI, I've had a pair mono-blocked for several years, both purchased new. Absolutely flawless construction and glorious performance. No issues with tubes and if rolling is your thing, plenty of alternatives to select from various online tube stores.  If you need to, do a little bit of research to properly match the 275 to the "right" preamp and speakers.  For a preamp, I use the PS Audio BHK tube hybrid. In my opinion, there is no better tube  power amp available near the price point.  While I have no issues with the amp, I would be cautious if purchasing used.  Over many years, I have worked with Audio Classics and never been disappointed.  They test and inspect the used amps and tubes.  In any case, if you're looking for a terrific tube amp, one that can be mono-blocked if you so choose, the 275VI is my top pick. There is a reason why the 275 is consistently on Stereophile's Class A list.  Cheers!

Build quality of the electronic internals is superb on the latest C2600. Beautiful. 

However they have skimped on the chassis. No flush mount screws on the top cover, which I find surprising on a unit of that price. 

I owned a MC275 MKV for a few years and enjoyed my time with it.  Trouble free and great sounding.   I have read that the sound signature of the MKVI has changed and they seem to be readily available on the used market, which would make me want a MKV.

I had a Mk VI in my system for a while and it ran flawlessly and sounded great. I just needed more power for my speakers. This list is posted on the Pearl Audio website and you might find it useful:
Here are some new things on the current MKVI:

• High Speed Sentry Monitor circuit will shut the amp off if speaker wires are shorted or there is a gross impedance mismatch to the connected loudspeakers.

• High Speed Sentry Monitor Circuit will indicate a failed output tube by illuminating the three small tubes Red in front of the defective pair of output tubes, greatly simplifying maintenance.

• Small tube illumination for amplifier operational status indication. The three modes are sequential: Amber on warm up, Green when amp is ready for operation, and Red if an output tube fails.

• A stereo power control cable combined with a McIntosh preamplifier allows for the small tube illumination to be turned on or off using the meter light control trim function in the preamplifier.

• Power control input and output allows one button on-off operation from the preamplifier when used in an all McIntosh system.

• High Power Mono operation, MC275 has been designed for 150 watt mono use and comes with the jumper strap to connect the two channels together for mono parallel operation into 2, 4 or 8 ohms.

• Auto Off switch allows control of the new Power Save Circuitry which will switch the amplifier to Standby Mode approximately 30 minutes after there has been an absence of an audio signal.

• New Style Patented McIntosh speaker binding posts produce the very best connection with all speaker wire types and terminations.

• Higher Audio Performance Achieved. There were many subtle changes in circuits, and component values that were relatively unnoticeable by themselves, but totaled up to significantly improved performance.