Wet as in the opposite of dry.
Tubes can add significant 2nd harmonic distortion, this imparts a sort of depth/warmth/fullness to sound, wet amps have a lot of this, dry amps less.
Warm get’s used as well, but it’s also used to describe a frequency response change which this isn’t.
Wet - A reverberant sound, something with decay. Opposite of Dry.
Yeah, typically planars pair better with solid state and some hybrids don’t seem to really make much difference with planars. I did recently receive the xDuoo TA-20 from Drop and was pleasantly surprised how well it paired with the HiFiMan HE-4XX which was unexpected with a planar. Other reviewers on Drop noticed the same nice performance. However my LCD2 Classics and Ether CX sound kind of thin on the TA20. As expected dynamic drivers sound nice with it using the HD58X and Fostex TR-X00s. I switched out the generic Chinese 12AU7 right away with some vintage US made RCA, Sylvania and GE tubes. These are still fairly easy to find around $20-30 for a pair.
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Besides my actual tube amp, the “tubiest” sounding amp I own is the Cavalli Liquid Carbon X. It has a smooth, soft sound that softens any bright headphone. It did wonders with my T5p.2’s. It is not an amp you would buy for its accuracy, but it does sound kind of tubey.
I am actually trying to sell this amp, but after writing about it, I feel like putting it back into the rotation. 
Some tube amps are designed with planar’s in mind take the Headphonia 2A3 for instance it was designed to work with HiFiMan Susvara’s…what a combo? 