I never recommend doing all the necessary mods to a PAS3 unless you absolutely have to have a preamp in that chassis.
The power transformer in a PAS3 is crap. The circuits were only passable (PAS) 50 years ago.
For the cost of all the upgrades (I've done severals PAS3's with $1000 worth of mods) you can buy the superior SP10 or SP12 preamp for less money.
At an absolute minimum, for a PAS3 to have decent performance, I would recommend nearly every mod on my website
To come close to the performance of the SP12 I would recommend the regulated power supply, the PAS-AIKIDO line stage, and PH10 phono (if you want phono).
That is items # 2,3,4,6,18,19, 28, plus new tubes # 24 & 25, so just the parts alone add up to $512. If you need them assembled and installed,
it's right about $1000 not including the cost of the original PAS3. A decent used PAS3 is about $300.
You can buy an SP10 kit starting at $650. For an assembled SP12 including phono the price is around $1300.
On my own rating scale of 1 to 10, a PAS3 is a 1 (comparable to a used $300 preamp). An SP14 is a 10 (comparable to a new tube preamp in the $3000+ range).
A modified PAS3 with the premium mods would be about a 7, and SP12 would be about an 8.
just my own opinion, of course . . .
(but this is after 25 years and at least 25 different preamps, and having modified at least 25 PAS3s, and having built at least 25 SP12s and SP14s)
just to add more insult to the injured . . . I'm also NOT a fan of any Audio Research gear. Their power supplies are so complex they destroy the tube sound.
So what DO I like that I've owned? somebody is bound to ask eventually . . . CJ PV8, PV10, PV12. Marantz 7, Luxman CL34 (both from 30+ years ago).
If I had to buy a new preamp, it would have to be a minimum of a Cary SLP-03 ($2000) or a BAT VK-3ix ($2500-3500).
But I'd also be happy with my own SP10. I'd take my own SP14 over any preamp at any price!