Never played one but the scoop is that they sound great.
ps: Even thought it's only 20 watts you might want to consider an attenuator so you can really crank the amp. That's how early NMV Marshalls sound best.
In terms of NOS stock - I think this guy is the king
There are machines for testing tubes. I'm not 100% sure everyone agrees what results really mean in real life applications. But certainly those machines test for a certain level of functionality.
Anytime a player gets to be there when the tech or builder is doing stuff, there's a huge benefit.
I had a Super Reverb that when I put in new power tubes, I was there at my tech's and we tried different settings. We got it really bang on and that amp always sounded great.
A great tech or...
I thought my '68 50 watt Marshall plexi was the ultimate amp but I kept reading about the big cleans that only the 100 watters have.
Bought a '72 Super Lead last year and everyone was right. The 100 watter is THE vintage Marshall to play.
ps: All made possible by Mr. Ho, creator and builder of...
You'll be getting advice to
1) Skip the Marshall reissues - get a Germino or Metro clone.
2) There has been a bit of a revolution in attenuators recently with the Ultimate Attenuator (there are even better units out there but they are VERY hard to get). So while your THD may help, I doubt you...
My buddy was hugelpressed with how versatile my '72 Super Lead is. As he said,
"hey - you can even play country with this amp". He was Super impressed at the range of the amp. I use an Ultimate Attenuator to control volume and run the amp way up.
That SR was the biggest amp disappointment of my life. I retubed and redid the preamp tubes also but the amp just never sounded like a great SR should.
My first SR was incredible. I sold it cause I couldn't deal with the 4X10 combo format. And then I got the head/cab idea and bought the chassis...