Reddit mentions of Nobsound HiFi 6J2 Vacuum Tube Preamplifier Stereo PreAmp Digital Treble&Bass Tone Control (Silver)

That SX-780 is a sweetheart. I am surprised you had struggles with it. The SX-x50 line up is debatibly better but I own both and have not heard much of a difference. I am currently using a SX-1250 (also own a 1280). Both recapped/diodes/transistors/etc etc replaced with high quality Nichicon, Wima, and Kemet caps. The difference is amazing.

If you were struggling with imaging/soundstage can be worked with in the pre-amp. Not sure if you can defeat the preamp in the 780 but you should be able to zero it out at least. At that point you could go tube pre-amp and really have some fun.

While budget I have something like this that I combine with the preamp on the 1250 (run in line with the aux input). Not necessary but really added a bit of warmth to the sound. I'd give that 780 another shot. After the 80 series it turned into black plastic crap minus a few choice pieces. https://www.amazon.com/dp/B01NBM5CTN/

Yeah, it works great with my setup really gives my bass more depth.

Turntable > solid state phono stage > passive tube amp > active speakers (heos 7 hs2).

This is /budgetaudiophile right? But seriously I had a little bear too and it crapped out on me as well. This one is highly rated and I suggest it with upgraded tubes: https://www.amazon.com/gp/product/B01NBM5CTN/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1

Ignore the whole "not for turntables" thats what the solid state preamp is for the passive is just for further control of your sound stage.

Below $300?

Well somebody is going suggest you don't need a sub just yet and suggest the JBL LSR305's. So let's get that out of the way.

My go to suggestions keep evolving as newer gear comes out. Currently I'd say:

1. BIC DV62SI for speakers. Well reviewed, but for some reason not many professional reviews. I have no idea why. I'd also suggest the Fluance SX6, because I have a pair and like them, but a well respected reviewer didn't, so take it for what it's worth. They need a little time to open up, but I've found them quite nice for their price, with a tweeter that can run hot but be tamed. Another popular choice is the Pioneer BS22, which IMO is too muted in the highs. It's all dependent on the listener, but they are often on sale under $100.
   
   https://www.parts-express.com/bic-venturi-dv62si--6-1-2-2-way-bookshelf-speaker-pair--303-402
   
   
2. Monoprice 12" Subwoofer. It's $99. There are better subs out there, but in at least one blind test it beat out pretty much anything in the under $200 category. For the price, it seems to be one of the better subs you can get.
   
   https://www.monoprice.com/product?c_id=109&cp_id=10906&cs_id=1090602&p_id=9723&seq=1&format=2
   
   
3. Don't laugh...NobSound Mini 50x50 BT amp. This thing looks like a joke. But I just got one on a lark and found out it's honestly capable of driving many different speakers at pretty decent levels. It's not going to make your ears bleed, but it can get fairly loud for a small room. It also functions as a decent headphone amp, has BT, accepts mini USB in, and all for THIRTY DOLLARS. It actually comes with a power supply and a bunch of connection cords as well.
   
   Edit: I got the updated version which does NOT have a 3.5mm out. It can't function as a headphone amp.
   
   https://www.amazon.com/Nobsound-Bluetooth-Amplifier-Wireless-Receiver/dp/B06Y67PZB1/ref=sr_1_3?ie=UTF8&qid=1509502947&sr=8-3&keywords=nobsound+mini
   
   I'll also add in the Nobsound Tube Preamp for a decent preamp with tubes and tone controls. Nothing fancy, but it's cheap and a good conversation piece.
   
   https://www.amazon.com/Nobsound-Vacuum-Preamplifier-Digital-Control/dp/B01NBM5CTN/ref=sr_1_6?ie=UTF8&qid=1509054033&sr=8-6&keywords=tube+preamp
   
   After that just add some speaker wire and you're golden.