Choosing an SUT?

I too found the Bob’s Devices Sky SUT preferable to the Herron’s JFET stage (though it is a very good JFET stage). As others mentioned, there are loads of SUT options, each with their own sonic profile. The Bob’s Devices are rich, robust, and fleshy sounding - on the warm side of things. They work well with a wide variety of cartridges.

HOWEVER, the thing about SUTs is that it can be tricky to balance optimal gain vs. optimal load if you’ve got one of those cartridges that asks for a relatively high ohm # for loading versus its output. This is somewhat the case with your Zephyr. Asking for more than 470 ohms load (others have mentioned it may benefit from a higher #) from a 0.4mV output is tough. You’re either going to be on the low side of optimal for gain or for ohms. Some folks are OK with less than 4 - 5 mV coming off of the MC step-up stage but I don’t like it.

One way to combat this is to find a phono stage that allows for flexible loading above 47K ohms in MM mode. Unfortunately this is more rare than it should be. I know the Rogue Ares allows for this (47K, 100K, or even 1 Mega-Ohm MM loading!) and so it can be a good partner with SUT for the usual "tough for SUT" crowd like the Benz non-magnetic ruby plate cartridges and (apparently) your SoundSmith. The Ares MM stage will be a slight step down from the Herron, but might be comparable to your Chinook. Then again swapping phono stage is a lot of effort just to try out a SUT!

One more consideration for SUT is that you will need a very short run of shielded low-capacitance RCA cables coming OUT of the SUT. If you want to hear cables make a huge difference IN A BAD WAY, then swap in some large gauge fire-hose braided geometry interconnects more than 1m long. Also you might have to deal with ground loop hum gremlins. The things we do for love (of analog)!

I’m looking at the Zesto Andros Allasso which has variable gain and load. Thoughts on that device?

It is quite likely a nice SUT, but the Allasso has a LOT of loading options you'll probably never use with your SoundSmith. It gives you the option to load DOWN your cartridge, which (as per manufacturer recommendations of 470+ ohms) is the wrong direction for your SoundSmith. You'd probably end up using either 1:8 or 1:12 in the 10th loading slot, which is the same net result as any regular 1:8 or 1:12 SUT *without* those loading options. Now if you could plug it into an MM stage with its own loading options up to 1 Mega-ohm, then the Allasso would give you a rich array of loading selections appropriate for your SoundSmith - but of course you'd have to recalculate the loading slot table to see what they remap to for each gain ratio!

With your Herron, I'd wager that its JFET stage is still your best option for that particular cartridge. You can still experiment with different loading plugs, and tube roll to capture some of the extra "body" that a good SUT would typically impart. That's what I heard when I hooked up a Bob's Devices Sky to the Herron, though my cartridges had 5 ohm coils and liked the ~129 ohms net load!