Which would you choose?

Could just buy a modded PAS-3 , still sounds better than most preamps.

I always like Krell pre-amps when I hear them. VEry dynamic lively, clean, transparent.

Have not heard Pass but would expect excellent results there as well, though some difference (warmer sounding perhaps?) than Krell.

For a vintage system like that, I would go in thinking Krell would add some very nice ingredients into the mix and help work towards a more neutral sound. Pass might as well, but less familiar to me.

You call those vintage components? Seriously?

Shakey

Even though this is a second system, I'm sure you want good synergy between the amp and preamp. For unbalanced (RCA) connections, there is very little risk of a problem, however, for balanced (XLR), the SA/3.9e will be very limited for a properly matched preamp. The input impedance of this amp is 50K ohms unbalanced, and 600 ohms balanced. The input impedance of the amp needs to be 10 times greater than the output impedance of the preamp for proper matching. So, for unbalanced, preamp should be below 5K ohms, and the vast majority of preamps will meet this spec. For balanced the preamp should be in the range of 60 ohms and below, and very few preamps will meet this spec.

Looks like the Krell KRC-HR should match properly with either connection, however the Pass Labs X-02 will match for unbalanced only. It's balance output impedance is 750 ohms, much to high for the input of the SA/3.9e.

BTW, back in the day at a CES show, heard Nelson say the SA/3.9e was the sweetest sounding of all the "e" series amps.

Thanks for all the responses so far. I appreciate them.

The Threshold 3.9e is a very sweet sounding amplifier. It also packs quite a wallop for its size.