I've had my Mk 4 for a few years now, I think Boogie stopped manufacturing them shortly after I purchased it. I had tube trouble with it right from the start. I was forever swapping out tubes and the sound was really inconsistent, good one day, fizzy and thin the next. I had read that the reverb on this amp was mediocre so I wasn't to concerned with how weak it sounded. I did my fair share of reading on this board and finally ordered the Premium Package from The Tube Store. What a difference. My Mk 4 finally sounds like it was meant to. It has a huge, organic sound and the reverb is 1000% better than it was with the Mesa tubes. Tone is a pretty subjective thing, but in my opinion this tube set (=C= 6L6GCs and the 7025s) are superior to the stock Mesa tubes.