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

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 

@georgehifi 
I'm pretty sure you got taken to task on that jazz already. No matter how you cut the cake, a buffer is going to provide more ideal impedance matching. You seem to love banging on about Nelson Pass. Maybe you should go watch his BAF lectures where he demonstrates the differences between a 25 ohm source and a 600 ohm source. It's not an insignificant difference. And you're talking about putting upwords of a few thousand ohm on the amp's input. It makes a difference. That's why Nelson has never once produced a purely passive pre-amp. 

Your really just not worth the effort, you live in denial kosst, and question everything Nelson Pass tells you. I’ll post it up again just in case you missed it.

Nelson Pass:
"This preamplifier flows from a commitment to create the best sounding product: a simple circuit with the most natural characteristic.

Unique to this preamp, patent pending, is a volume level control which combines the best qualities of a passive attenuator and active gain circuitry:
At the 3 o’clock volume control position, the Aleph L offers a direct path from input to output.
The only component in the signal path is wire and switch contacts.

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."

And this as well from him just for good measure.

Nelson Pass,

"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.

Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.

What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (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."

Hello @georgehifi

We've had a similar discussion about preamps:
https://forum.audiogon.com/discussions/bypassing-a-preamp-with-volume-pot-in-amp
I knew I've seen that Nelson Pass quote before.

I have had great success "bypassing" a preamp with a Khozmo stepped attenuator using Z-Foil TX2575 resistors inside an amp.

I ditched the volume attenuator completely in my current setup and only control the volume from the computer. I prefer to lose a few 1s and 0s from the computer volume than to add a preamp that brings so much favorable and unfavorable distortion/coloration from tubes, resistors, capacitors, regulators, diodes, interconnects, power cords, etc.

Who needs an preamp when you have a Pass XA25? (jk) This amp is amazing.