Passive preamp

How does one avoid impedance mismatches?

Using the accepted impedance matching ratio of 1:10 or higher.
A 10khom series/shunt passive preamp is the best most versatile value to go for as it has:
10kohm input impedance
2.5kohm max output impedance.

The source (dac ect) should have an output impedance
10 x lower than the input of 10kohm passive pre (which most are) so sources with 1kohm or lower output impedance are a match.

The power amp/s should have an input impedance 10 x higher than the output impedance of the passive pre 2.5kohm (which most are) so amps that are 25kohm or higher are a match.

The above is good for "nearly" 100% of systems out there

Good practice also is to use low capacitance interconnects (100pf per ft or lower), which most good quality ones are.

Tube sources should be questioned as they have output coupling caps that "may" not be large enough value, and easy fix though.

So your Classe with 27kohm input will be fine for a 10kohm passive preamp like mine.
https://forum.audiogon.com/discussions/lightspeed-attenuator-best-preamp-ever

Cheers George

I have a passive that I ran in my system for a long time and really enjoy it. However right now I am enjoying the distortion that a tube  pre has to offer. If you have the gain,a passive is the next best thing to straight wire. 

If you have the gain,a passive is the next best thing to straight wire.

Hit the nail on the head, it’s the most transparent/dynamic/uncolored way of getting the sources signal to the poweramp/s

Quote from Nelson Pass

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.

Cheers George

@georgehifi 
I have sent you an email asking for pricing info for the Lightspeed Attenuator. A friend and I will test it seeing as though the reviews have been rave. Excellent work, George. Incredibly excellent work. However, when I am having a party (I have them quite often) and would like to change among sources, it would be great to have a passive tube preamp that can accomplish this. I have read about the Schiit Freya among others. Would you make any recommendations? Thanks so much for your time.

I own the original version of the Schiit Freya and it can switch immediately (the newer Freya + "improved" version shuts off the tubes when one of the other outputs, passive or FET, are utilized...so you have to wait for the tubes to warm up if switched) between the 3 modes...passive sounds fine but the tube mode sounds better for my tastes. Just more "there" there. Switching among modes keeps the tubes honest since if you notice a degradation of some sort with the tube mode you simply replace the tubes.