The old one is fine on solid state, it's a very detailed technical headphone and I think it sounds better on solid state. You can EQ in a bass shelf if you want that, that's the most significant audible difference you'll get from a tube amp.

Like many dynamic drivers the OG Utopia has an uneven impedance curve that peaks in the bass region, this results in a boost in the bass if used with an amp with a high output impedance, like many tube amps have.

Unlike Sennheisers though, where due to the high impedance the boost is subtle, with the lower impedance Focals it's quite a lot, and it's a mid-bass boost rather than a sub-bass boost. This is very audible, but can be leaning towards boomy, as it's mid-bass then with a sharp roll off into the sub bass. If you just EQ it yourself, you can get the bass rise but with less boomy mid-bass and better sub-bass extension.

The 1-1.5kHz rise is a feature of many Focals, that region is identical in the old Utopia and it's also there in the Clear. You can EQ it down, or alternately, smooth it out the other way by EQing up around 2kHz, this is what Amir did with his EQ of the OG Utopia and I think it works well. The new Utopia does seem to have smoothed over that dip at 2k a bit though. I think maybe you get a bit more soundstage if you bring it down, but also a certain thinness, while it has that more typically Focal forward character if you leave it in but smooth it out at 2kHz. Worth trying it both ways.

Interesting that you feel it needs more treble, this was the major change from the old one, they cut the treble, similar to what they did moving from the Clear to the Clear MG.