If you are looking for the tube sound(warm,lush)you're not going to get it with the LS-25mkII. The LS-25mkII is more of a hybrid than a true tube preamp and will never sound warm and lush. It will however do all of the other things that you've mentioned(bigger, deeper, wider etc.) The 6h30DR's will only make it do it better, more dynamic as well but will not change the sound to warmer in the sense that you seek. I have the LS-25mkII and I'm happy with it as I like the ARC sound. It has never sounded dry and sterile to me. I've heard that analogy before from others but my question always was dry and sterile compared to what?
Again, if you are comparing it to a true tube preamp, then yes it is going to sound different but I don't know if I would characterize that difference as dry and sterile.
Finally, I would think that you would be getting a very good sound with the setup you have as ARC preamps mate very well with ARC amps particularly the VT-100mkII which was rated very good tube amp.
Just my two cents worth.