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

It doesn't matter one iota where the volume control is positioned here, these figures have a perfect impedance match regardless of volume position. As do 90% of sources and amps out there when mated to 10kohm passive volume controls.

This statement is false. Its not wise to assume that 90% of all sources can drive a 10K load.

The position of the control does affect the sound in many cases (although most people don't get to find this out because if a digital source is used, the output will likely be clipping the amp). So an audition with something like a tuner that makes less voltage and so won't clip the amp will reveal the issue.

Audition is the bottom line- if everything is set up correctly, a passive will work fine.

So the only reason for a lack of bass in this case (if correct) is a coupling cap that's too small, and that 100ohms source output impedance is rising at low frequencies which "would" roll off the bass.

This conclusion is based on a false assumption, and so is also false. Its highly unlikely that so many sources would have a coupling cap at the output that is too small.

Correct me if I'm wrong, but isn't the connection between components somewhat similar in behavior to that of speakers to the amp? Is there anybody out there designing amps or preamps intended to be driven by sources in the thousands of ohms? I thought the whole point in there being massive impedance differences was so that the preceding component would see something as closely resembling a purely resistive load as possible. I'm sure 5kR would make no difference at all to my F5 with it's 100kR input, but lots of amps have 30-47kR input impedance. If you start showing those things 5, 10, or 15 kR sources, they're not going to behave well. 
Bottom line is that you pretty much have to build your system to play well with a passive volume. Throwing them willy-nilly at random systems isn't going to get good or consistent results. 

Its not wise to assume that 90% of all sources can drive a 10K load.

This is picking hairs and scare mongering, and just selecting a worst case scenario. 

90% of amps are 33kohm input impedance or higher. 

Very rarely is an amp a pitiful <10kohm, which even many tube preamps won't drive let alone passives.
In fact most amps are >33kohm or higher to 100kohm or more. The loose industry standard input impedance has always been 47kohm, though it's not adhered to. 
  
So the impedance ratio of those 90% of amps regardless of volume position with a 10kohm passive will always be better than the industry regarded minimum ratio of 1:10 source to passive for the input, or 10:1 minimum for the output, passive to amp.
 
And with the voltage of most of today's sources again are higher at >2v,  and the input sensitivity of most amps are <1.5v, the only thing that could "clip" is the amp with too much volume from the passive volume control, so more gain from an active is not needed, in fact it's a curse as then the volume control can only be used right down low in many cases.

Actives are not needed unless you have an amp that is 10kohm or less and/or it's input sensitivity voltage is higher than the source volt can give out.
  
Or the other contentious reason to use an active pre is, if you like the colouration of active preamp, to me it's a bit of a ban-aid fix for something else that's not right better to fix it, as no active preamp sounds like the proverbial "a straight piece of wire" only direct source to amp can do this or second in line a passive volume control.    

Cheers George 

And here we are again debating where it's best to put voltage gain.... 

We have the voltage from most sources at more than >2v.
Most poweramps only need less than <1.5v in, (some even 0.5v) to make them give full wattage output to the speakers.

There is no debate at all, as any **** can see.


Cheers George