critique my setup -- weak link? best way to improve?

A separate phono preamp, definitely.  Schiit is a good recommendation for a budget price.  If the speakers *must* stay on the credenza, then get some desktop stands for them, they'll sound much better. IsoAcoustics are popular.  Otherwise separate stands will improve things a lot.  Between those two fairly inexpensive changes I think you'll get more enjoyment out of the current system.

If the speakers *must* stay on the credenza

I pretty much ignored this part. If you want good sound don't let the furniture determine it, proper stands and placement are literally the easiest and cheapest tweaks of the system and the best bang for the buck. 

If you are forced and willing to compromise with the placement, you are probably not crazy about achieving the best sound. And nothing wrong with that....

Based on your post focusing on your analog needs If you don’t already have the speakers isolated from your sideboard look at Isoacoustics mini pucks at a minimum. Also a standalone phono preamp from the likes of Schiit or others within your budget and an updated cartridge at least in the $200-300 range or more.

An upgraded Preamplifier more in line with your Poweramp would help your analog and digital quality.

Re Seeker, nearly all preamplifiers ever made operate in Class A, and many/ most modern tube Preamplifiers can drive a 10K ohm load easily (which is to say their output Z is 1K ohm or less). I agree those are parameters that need to be checked out before you buy a tube preamp to drive an SS amplifier. You want input Z to be 10X or more higher than output Z.

There are some well proven bookshelf   style speakers that are affordable and you appreciate a much smoother sound after their broken in after 60 hours or so. Those speakers are the Wharfedale Lintons. They are usually on lower price around Xmas /black Friday specials.