I own the non-signature version of the iDSD. I use it as a dedicated DAC feeding my Stax tube energiser (driving SR009) but also as an all-in-one amp/dac to drive my LCD5/LCD2fazor/LCD2ClosedBack. In both uses it is brilliant. The DAC is very neutral and the head amp section has more than enough power to drive all my headphones. I use the 9db gain (middle gain option)
Just last night I decided to use the LCD2ClosedBacks for a listening session. I started off with SS and then later switched to tube mode + (there is a mode in-between as well which is a tube-hybrid-type mode). The tube mode + mellowed out voices, took the edge off leading notes and made magic with the soundstage, increasing width and depth. So to answer your question, yes there is quite a difference between SS and tube performance to my ears. I do prefer SS on the LCD5 though as it is a very technical and detailed headphone and I prefer the sound fast and neutral.
I recently had the opportunity to demo the Chord Dave, Holo Spring, Holo May and the eye-watering expensive Nagra Tube DAC. I walked away even more impressed with the iDSD considering cost to performance. Its small footprint takes up little space and you can tweak the sound using the various filters and tube modes to your liking. It also has MQA decoding if you buy into that hype.