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

So, impedance matching, and gain, are both important ?

An active preamp is an active buffer due to its gain, which largely resolves the issues of passive buffers.  Lots of circuits in the world have an active buffer to better mate an input to an output stage. A stereo system is no different.  I have yet to find a passive buffer/divider that does the job as well as an active buffer, mostly because of the much-lower output impedance in the latter, which prevents stage modulation.  It's also very easy to witness the benefit on an oscilloscope.

SMC amps designed to work great with just volume control so no preamp is best. 
If you find used McCormack Micro Line Drive that offers passive and buffered active preamplification, you will forget about searching any other preamps.

@atmasphere "As per usual, George omits the rest of Nelson's words **after** the quote he usually trots out. I've included more of that text above, seems to me not for the first time on this thread."

Looks like @csmgolf  (Cris) found the perfect description to how @georgehifi plays the game, cherry picking.

And as clearly seen, his other dirty trick is ignore questions by raising the issue of the other side being a fuser, snake-oilist, etc.  Then, he can dismiss that person out of hand, on anything audio and everything else in life, forever.  However, trying that with Cris and myself, George only exposed his own ploy as neither of us fall into any of those descriptions

@mrdecibel , IME, impedance matching (not matching as in "equal" but rather low to high as you move down the chain) is more important than gain. If it were not, you could simply increase the output of your source...to 4V or 6V, but that alone doesn’t seem to cut it for many listeners. Most sources (even those with sufficient voltage gain) cannot suitably handle the impedance changes that occur as the signal passes through the volume control and through the interconnects. Aball does a nice job of discussing the benefits of buffers a couple of post ago. There are trade-offs, and personal preferences, as always. Having less circuitry can sound better to some, but only if the ancillary consequences are not deal-breakers.

BTW, Steve McCormack (SMc Audio) makes one of the best sounding buffered preamps you can buy - the VRE-1C. That unit is the culmination of a career’s worth of research and trials going all the way back to his early Mod Squad/McCormack days with the Mod Squad Line Drive, TLC- 1 (Transparent Line Control), and Micro Line Drive units. You can order the VRE-1 as either a unity gain unit, or with 6dB gain through the high-end Lundahl transformers he uses. Most order the +6dB version as Steve believes the differences between the 0 and +6dB gain versions are virtually undetectable. I have tried both in my (very similar to VRE-1) unit and own a 0dB gain version. With 1.5M balanced cables to my monoblock amplifiers, I cannot hear any loss of signal, bass, dynamics, staging, or anything, compared to the +6dB version.

Regarding Steve’s McCormack amplifiers, I believe all of them originally had a 100K ohm input impedance, up to the DNA-500, which was designed with a 10K ohm input impedance. The upgraded/revised versions which have balanced inputs, all have a 10K ohm input impedance, which would generally be difficult for an unbuffered passive volume control to drive without some sort of signal abberation. My own trials with passive and autoformer preamps have mostly been into my Clayton amplifiers that have 100K input impedance, and the result has always been similar....an attractive purity but also a flatter, less dynamic, less full-sounding, somewhat bass-shy, and overall less satisfactory sound compared to what I hear through my active buffered (unity gain) preamp. BTW, I heard Steve does an outstanding job modifying/upgrading the Micro Line Drive unit so you could consider that. He also works magic on the TLC-1!