impedance match of preamp and main amp.

Good stuff, as usual.

Its good to know detailed impedance measurements when available, but in lieu of that a good rule of thumb seems to be to use an amp with 60Kohm or higher input impedance to be safe with most tube amps. Slightly higher than the 10:1 minimum (as applied to worst case measurement at particular frequency) usually cited is a good and safe insurance policy.

FWIW, you will find most amps that explicitly mention or support use with tube pre-amps tend to have at least 50-60Kohm input impedance spec, often 100kohm unbalanced and twice that even balanced, which is usually preferred for the best and most consistent results.

Thanks for all your help. I think I will look for an amp that matches my preamp. Maybe one of the BAT amps.

Cturnem, sounds like you want to keep your Bat preamp, and that Mac looks to be a nice amp, if your keen on owning it.

Your Bat preamp is a low 410ohms output impedance at most frequencies and into the Mac it has a good ratio of 1.24, the problem is in the bass, and as I said above this is an easy fix if you get a tech to change the Bat's output coupling capacitors to 4 x larger in uF. Then it will be a perfect 1:24 ratio at all frequencies.
This job to do should be around $100-$200 then your Bat will also be a match for any amp you purchase in the future as well.

Cheers George

Thanks,
Georgelofi. I believe what you are saying, but am not sure I want to make the VK 51se non stock( due to resale value) I am running a Classe CA 400 with the Bat. I think it is 70k input impedance. I am running B&W 800D speakers. I have no reason to want the MAC, except from reading all of the reviews, and it is more powerful. I have also considered the BAT VK 600Se. I have never owned anything MAC. What do you run?

I like big solid state, biased hard into class A with huge power supplies, and that has bi-polar output for great current delivery.

The Classe CA400 you have is no slouch either, I would be quite happy owning that even though they are Mosfet output.

Your Vk 51se should be happy into that 75kohm input impedance.

The B&W 800D's are a bit of a bitch to drive, if you can understand the test here.
http://www.stereophile.com/content/bampw-800-diamond-loudspeaker-measurements

I would be more likely to go the big class A bi-polars for these 800's like what I use. Things like the big high class A biased Krells, Mark Levinson's and such that use bi-polar output stages, for good current delivery for the 800's.

Cheers George