Which would you choose?

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.

those speakers demand a very high level of partnering equipment. they are exceptionally revealing and will not flatter inferior kit; they will expose it ruthlessly
since the phalanx can be made to sound a bit fierce at the top by ill-considered partners, i would suggest a more ethereal sound than that usually offered by krell or pass. i have found YBA electronics to be a good match, especially their cd player cd lecteur 1, available in one to three-box configurations, and the passion power amps, again various specification up to 1000 monoblock. (don't stint on the power because the phalanx aren't the most efficient)
i like the yba because they truly seemed to benefit from the massive soundstage the phalanx are capable of producing, combining to give an epic, huge presentation when called for, or breathy realism when that is what's required
if you can sense me getting a bit misty eyed for my own phalanx you'd be dead right. last year i traded them for focal stella utopia, and you could say they've left a hole behind.
i'm now trading back to nola because now you i've lived with well implemented open baffle speakers everything else just sounds like half the music is being left in the box

i would seriously consider mixing a valve pre-amp with solid state electronics...or even valve pre-power. i just worry solid state pre-amps are going to sound too bright in that setting
finally it's worth paying closer attention to cabling than usual because of the separate crossovers. i found solid core cables worked best for the bass, with van den huul for the mid and tops...but i'm buggered if i can remember which specific cables they were... sorry, i may get back to you on that

There are two Threshold Fet 10's listed here. They would be a great match with the 3.9.