Opinions on Musical Paradise MP-501?

@6sj7 Thanks for your input, I'm very glad to hear this :D 

I'm more of an analogue guy, but do some digital stuff.  I plan on getting an Oracle Origine Mk II once I can afford one...I currently have a Pro-Ject Debut Carbon with 2M Blue and acrylic platter.  As for a DAC, I plan on getting the Schiit Modi Multibit, as it seems to suit my needs the best.  I certainly enjoy the DAC that's in my Marantz CD6006, so that won't be changing for my CDs anyway.
The MP-501 would be driving my Focal Aria 936s, which is about the most I can ever see myself spending on speakers.

All I can say is that I have an MP-501 V5 (the latest version) and it is a very fine, and very affordable piece of kit. I bought it having had a very good experience over a year or so with the smaller MP-301. There is a dramatic improvement and, assuming the 501 lasts, I expect to keep it for many years. As it is so inexpensive it would not be a tragedy if it did't last, but it seems well-built and in the course of 5 versions, the designers have clearly improved it.

Like the PrimaLuna the 501 is also auto-biasing. 

I am not sure that you need be concerned about the points your raise at the end of your post. For decades valve/tube amplifiers have not had these features and, when the tubes fail, all is well after replacement. I'd suggest that of all the things you need to consider when making a purchase, your concerns need not be amongst them.

The 501 uses KT120 power tubes which are current production and should last a long time. It uses readily available and very cheap driver tubes. I have bought lots of various types for very little on eBay and have enough to last me out now!

I listening to the 501 now and am enjoying a fabulously detailed, rich, three-dimensional impression of the Vienna Philharmonic playing Brahms. It may not matter to you but the 501 looks beautiful too.

A lot depends on your budget. I took the decision to go for the (cheaper) 501 and buy the very best DAC I could afford (Chord Qutest). This strategy worked for me.

Hope this helps.