Esoteric need a pre-amp?

You only need a (good) preamp if you want to hear all of the beauty the machine can unleash. If you don't want all of the magical midrange, go w/o the preamp.

The X series got upgrades but lost the volume control

With K-01X Esoteric dropped the internal volume control feature that was formerly in K-01. While I don't need that feature, I do wish the K-01X had the ability to change between the selectable up-sampling and filter options via remote control. That would be useful.

I have a K-01 and VAC sig SE. I was initially using the K-01 as a preamp to drive my power amps directly and it worked great (you have to change the esoteric settings for it to act as a pre). Ran it this way for many months. Then I inserted the VAC pre. I preferred the sound over using the K-01 directly as a pre. It just seemed to make the sound richer and took off the digital edge. I will say if you don’t use vinyl and can avoid a pre this is a tough decision given the high cost of a good preamp. Hope this helps