I'm using a Wadia 121 DAC with built-in volume control and am quite happy with it (list is $1200, but I think you can find them for more like $800 now new). Source = MacBook Pro mostly streaming Tidal lossless. Feeding a Bel Canto Ref500s amp and Linn Majik 140 speakers.
Recently, I became curious about whether using an analog volume control would make a difference. I'm currently trying out an iFi iTube, which is a pretty interesting little device that's received great reviews. You can use it as a tube buffer or a preamp. for starters, I'm using it as a tube buffer. The Wadia and Bel Canto both have balanced and RCA jacks, so I am using the balanced to go direct from DAC to amp, and the single ended plugs routed through the iFi. I can then use the switches on the amp to toggle directly from using the iFi to not using it.
So far, using the iFi purely as a tube buffer, I find an extremely subtle improvement. So subtle that I'm not sure it's actually an improvement, but I'm still burning it in, so I'm going to be patient. Once it's more burned in, I'll experiment with using it as a preamp.
I would recommend that people looking for budget DACs and components check out the iFi micro line up. From what I've seen so far, they are pretty cool little pieces that punch way above their weight class.
Anyway, back to the original questions, if I were in OPs shoes, I'd be considering the following:
Wadia 121 with digital volume control; ditch your preamp
Centrance DacMini (now on super sale on Amazon for about $500!). They make nice DACs for the money
Schiit Bifrost or Gungnir - but you need a preamp. Could use your current one or buy something like the iTube referenced above for $300
iFi micro iDAC or iDSD DAC.