How much do I need to spend to get a preamp that sounds better than no preamp?

I just sent my Axiom back

mrdecibel
  A veil has been lifted, the colorations are gone, and nothing is missing from using my 2 expensive preamps
  Thank you George for steering me in this direction, as I always thought an active preamp ( with gain ) was absolutely necessary

No problems MrD, you've obviously got a good match if your hearing all that, now you've seen the light and just turning that passive up some more compared to the active. As Nelson Pass said:
"  Really - I’m being serious here!. Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."

Cheers George

Per my earlier post above on trying the Belles ARIA preamp in my setup, a little update.

I borrowed the unit from my kind dealer for just short of 3 weeks, a period of time I found sufficient to evaluate its impact. Overall, the encounter hasn’t led me to deviate from my initial "preamp, no thanks"-stance - via my specific setup - but it’s not without acknowledging some qualities with the ARIA preamp in the mix.

The ARIA imparted more energy in the upper bass region, which seemed to smoothen (i.e.: make more flat) this area compared to the SOtM DAC-direct connection, and moreover spatiality and/or the sense dimensionality was more pronounced/convincing, which was especially apparent during live applause sequences that seemed to emanate from a plane just behind the speakers with striking realism - quite impressive. The uptick in upper bass energy also made voices come through with a bit more solidity.

The areas that detracted from the otherwise positive impression of the ARIA were a variation of factors, the most pronounced of which was a sense of filtering of the sound. I felt this robbed the presentation of some "information" or very fine details. The sphere of sound also appeared to take a little step backwards which I found took away some of the tactile and enveloping feel of the music. Lastly there was slight darkening of the sound, as if the top end had rolled off a bit.

I then, with the assistance of a friend, ended up thinking over some tweaks with the SOtM DAC-direct approach to address the upper bass prowess of the ARIA, and looking into some added gain and phase correction of the sub we found just what we needed here. Moreover the synergy with a different software playback program proved an advantage, leading to a huge and uninhibited soundstage, depending on the material; intimate recordings has a wonderful presence and focus, and large scale recordings open up accordingly (as if) outside the plane of the speakers. These tweaks gave us what we found the best of both worlds, and so rendered the ARIA moot.

I’m aware, or can at least easily imagine some may have found the contribution of the ARIA more to their liking overall, at least via my setup, particularly in light of the slightly more laid-back, more spacious and perhaps more pleasing presentation. As is - without the ARIA and with the tweaks mentioned - the plane of sound seems more direct, less restrained, and more enveloping and spherically even. There’s no lack of "meat on the bones," dynamics or life in the sound - indeed that’s where the ARIA-in-the-mix would fall short in most aspects.

Through all this what has amazed me is the chameleon-like character of my speakers, where what was previously considered to be limiting or signature-related aspects of the sound imparted by them turned out to be source or (pre)amp based limitations/character instead.

Just my $0.02..

The ARIA imparted more energy in the upper bass region, which seemed to smoothen (i.e.: make more flat) this area compared to the SOtM DAC-direct connection, and moreover spatiality and/or the sense dimensionality was more pronounced/convincing, which was especially apparent during live applause sequences that seemed to emanate from a plane just behind the speakers with striking realism - quite impressive. The uptick in upper bass energy also made voices come through with a bit more solidity.

The most common problem with operating a passive control is the loss of bass impact; in this passage you are confirming it.
What I don't get is why you stopped with the Aria. There are a lot of preamps out there, and they really vary! IMO/IME it is a statement about how bad many of them are that a passive system can keep up with them. Getting a sub adjusted correctly is always a tricky bit- and I know David Belles well enough to know that his electronics are really flat- so the bass you are referring to above was not being enhance by his preamp. It some other interaction, and setting up the sub to compensate is creating a synergy.
IME again, synergies are usually a situation where one weakness is played against another, resulting in something that *seems* better than either on their own. An alternative is that if you worked with stuff that stood on their strengths rather than their weaknesses, you create something that is even better.  I would try a variety of preamps and see if you can do better that the Aria, and be willing to readjust the sub in that light (since you were willing to do that to compensate for the usual loss of bass otherwise...). You might also look into tube preamps; IMO they offer more detail than solid state.

When I inserted a Hattor XLR passive (sans buffer) between my Oppo 105 and my then EP 100.2SE amps(newest AA amp on its way)  everything improved, especially bass

It could be that my  system  is 100% balanced, Isn't everybody doing that by now?