I only have one tube in my system that belongs to a MPI-4. After 40 years Audio classics did the replacement and rebuild. I heard the 220, 2200, 2300, owned an original C-22, used a 20 and an 11. I wasn't impressed. Now a C500 T I would keep if some gave it to me. But the idea of replacing tubes just rubs me the wrong way. It did when I owned 240's 275's Mc 60's and a MA 230. I got a real nasty taste owning Knight, Fisher and Scott pieces first. At least a Mac would usually last a year between tube changes. My favorite Mac tube piece is the 3500. I was bi-amping and having 4 3500's was just out of reason at the time. I would need 6 today. and with Home theatre 8 for the 3 front channels. I used 3500's with PA system, both indoors and outdoors. They were dependable, always sounded great, we changed tubes every year and performed a couple of up dates while in use. I drove Ampex capstan motors with 3500's to get variable speed control. Installed a couple of pairs to drive Concert Grands. Hearing Grands with the meters indicating 35 watts were breath taking. What I liked about the 3500 was how fatigue free it was. It had a 1/3 or 1/4 the distortion and almost 10 db more signal to noise of other Mac amps at the time. I thought two 3500's wiped a 2300.
I could own 3500's even with all the age and tube issues. It was that special. I could own a Frank Gow C2200,too, a remembrance of a close friend. The 3500's, C2200, and my old Ampex 300-2, with some original HS tapes would be memorable.