If you ask Nelson directly he will share his own awareness and support of his own customers running tube preamps with his Pass Labs amplifiers. He told me personally its quite common, and referenced awareness of those who run the same Cary tube preamp I run now, fwiw. It was common years back to see ARC, Cary, VAC preamps paired up sometimes with Pass Labs amplifiers.
He'll tell you, he's not a tube guy, and defers that business to others who really know tube components well, however... one exception, while not phono based...
and not exactly the same, Nelson did create his own version Korg NuTube Preamp kit for customers who wanted to experiment with it. I had conversations with Nelson when he was designing this and it was inspiring he was willing to give it a go!

