Amplifier upgrade

both the Pass X250.5 and X250.8 would be a big step up from the the A21 or the 8008.  The 250.8 is also worth the cost over the 250.5

I have the D'Agostino Classic Stereo in my system and it's a terrific amp! Was trying to decide between it and a Pass x350.5 when I was amp shopping.
The D'ag is slightly on the warm side of neutral with a very open sound, imaging is excellent, soundstage is noticeably deep. For a 300 watt amplifier, it's bass is not the powerful tight fisted variety but surprisingly a lot of texture and only shows its power when the music calls for it.
I do find that the Dag likes to be warmed up for a long time before sounding its best and this amp only gets slightly warm to the touch after hours of use. I remember my Pass X250.5 getting a lot warmer driving the same speakers.

My suggestion is to contact Mark at Reno HiFi. He is the distributor for Pass Labs nationally. He will loan you for a cc number an amp, preamp, or integrated. You can use it in your own system, the only way to make a reasonable decision. If you don't care for it, send it back. I recently purchased a 150.8 Pass amp and it has been one of the best audion changes I have made in quite a while. Reno gives super service and great deals on Pass equipment new and used. I am just a happy customer and not connected with Reno.

Since you already have a tube preamp & warm sounding speakers, I would go for a 2nd hand Boulder 1060. The Boulder is superbly built, natural sounding, powerful & neutral; an ideal match imho. And you can pick up 2nd hand 1060's for a relative bargain these days when them come up FS.

I have an ARC Ref 10 and it loves my Octave MRE 220's, which feed ProAc K-6's.  Immediacy and transparencey of very expensive SS and the realism and monster sound stage that only tubes can deliver.  Have fun with your hunt.  I've thoroughly enjoyed this kill from when I went on my hunt to upgrade my amps from the Ayre MXR's.  The Octaves smoke the Ayre.