The Soloist has a stepped attenuator not a potentiometer. That means you have a network of 24 resistors and you switch between them instead of having a continuously variable resistor (the potentiometer). It also means you adjust the volume in steps (the more the better).
What can happen is this: if your speakers/headphones are very sensitive and/or your speaker amp has high gain, you're left with too few steps in the usable volume range because it's too loud after the first few steps. That means you can't quite get the volume to the right level, the steps in the usable volume range are too big.
Now the Soloist also has a gain switch and that means you can bring the gain to an appropriate range to better use those 24 steps so that's not the problem it was before with the HA160(D) when you didn't have a gain switch. It can still happen that your speakers need a different gain than the headphones so you always have to press it when switching. As I said, as a preamp, not ideal for convenience.
I would still have the stepped attenuator for sound quality and reliability instead of a potentiometer though ... the Soloist with it's increased power and gain switch can drive any headphone, it's just not the right choice for the HD800, not unless you mod the HD800 and/or have a tube DAC for them. The Soloist is more an amp for the HD6x0 and the ortho's (LCD2, HE500).
Used Luxmans are rare but not unheard of. I can't help you there, keep checking ebay, audiogon, craigslist etc. You can also try to import a used one from Japan, that would be the cheapest way but more risky.
Also check for the Rogue Audio Cronus ... that's an integrated with a nice tube headphone out as well. If you're lucky you might find a used but still under warranty example for 1000$. Good luck!