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

I do not believe "gain" to be the main issue here.  IME, the reason some hear improvements in dynamics and bass when moving from a passive to an active preamp is due to improved impedance matching through the volume control and consistent current delivery.  An active buffer (with or without added gain) can make the source's job of driving an amplifier easier by allowing the source to see a high input impedance into the volume control and the amp to be fed a consistently low output impedance following the volume control.  With the high'ish output of today's sources (i.e., CD players at 2V and  DACs at 2V to 6V) gain is probably not the main reason people hear improvements due to active circuitry.  

mitch2

I do not believe "gain" to be the main issue here. IME, the reason some hear improvements in dynamics and bass when moving from a passive to an active preamp is due to improved impedance matching through the volume control and consistent current delivery. An active buffer (with or without added gain) can make the source's job of driving an amplifier easier by allowing the source to see a high input impedance into the volume control and the amp to be fed a consistently low output impedance following the volume control.  

  Nelson Pass tends to disagree, with his Aleph L, he says when the active stage (which you would assume to be first class) is engaged on this preamp it takes a hit "his word suffer" in sound quality (3 o’clock or more) from when it’s in passive mode before 3 o’clock.

Nelson Pass:
At positions below 3 o’clock, the volume control functions as a precision passive attenuator using discrete resistor ladders.
Above 3 o’clock, active gain is added to the output signal in 2 decibel increments, for a maximum of 10 dB.
As a result, you suffer the effects of active circuitry only when additional gain is necessary.

Cheers George

From Nelson's B1 Buffer paper;

Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.

This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa.

A lot of this is system dependent....i.e., electrical values of the source upstream, amplifier downstream, and amp-speaker interface, as well as which cables are being used and how long they are.  I agree there is a purity to the sound of a passive but there are more than a few of us who continue to believe something is missing when inserting a passive into our systems.

A lot of this is system dependent....i.e., electrical values of the source upstream, amplifier downstream

If we look at what the OP has there is no issue, 100ohm > 2v source and 100kohm 0.8v sensitivity poweramp.
And once again this goes for 90% of matches, only high impedance tube sources can have a problem or very low input impedance amps.

Cheers George