I have had a 4004 mk II for several years. A few years ago a friend lent me an ARC SP8 to try with it. Those who criticize audiophiles often assert that the differences in sound that we percieve among things like electronics, interconnects, etc. are due to expectancy effects. The theory is that we hear the things we expect to hear.
I inserted that little digression, because it was germane to my experience with this combination. I expected the touted sweetness, lushness, liquidity, etc. that tubes were supposed to bring to the system. In my system (MG-III's, Theta DAC, SOTA/SME V/Monster Sigma Genesis, Audioquest cabling throughout) nothing could be further from the truth. It was, in a word, harsh. It was thin and brittle, with alot of midrange glare. I quickly reinserted my Meitner PA-6i and it sounded great again.
Meanwhile, the SP8, returned to my buddy's system, sounded fine.
This has led me to two conclusions.
The first is that, based on this singular experience with an admittedly 20+ year old design, I wouldn't recommend it. However, YMMMV.
The second is that there are many things about listening to reproduced music that can't be explained.