I don't believe the Shindo is balanced, if that matters to you. I would recommend an audition of the ARC Ref 3 if you get a chance. Which Claytons are you driving? M-200's?
Your right on the Shindo, RCA only. Good suggestion on the ARC, since I had forgotten about that one and it does get good reviews and can be had at decent price. The MBL and Gryphon are on the expensive side for a tube sound in SS package. I'm going to run 300's, which are very neutral with a tubish sound.
You most likely will not have heard of this brand but it should be on a short list IMO.
Reflection Audio Quantum...it is fully balanced and uses a battery power supply. Stephen Balliet has done an excellent job with this product that competes with or exceeds the very best.
I have no connection to Stephen or Reflection but would encourage you to check it out.
A Clayton Pre-amp could be a consideration, but it also could lend to a leaner, dry sound verses using a tube based preamplifier for some warmth and musicality. I've seen more references of Clayton amps being used with tubes for realistic music reproduction than a SS pre. I don't see alot of SS pre's getting musical recognition without coloration regularly except the MBL 6010D, Gryphon Mirage, or Krell Evolution Two and look at their price points. From a price performance ratio it will be interesting to hear how the Clayton sounds relative to it's price as compared to tube pre-amps. The recent review of the 200's in positive feedback used a Cary 05.
If you go with a balanced preamp, it will help if it supports the low impedance (600 ohm) balanced line standard. This will reduce the effects of the interconnect cable dramatically, and free you to have the cables as long as you want.
There are very few tube preamps that support this standard (without frequency response problems). The traditional way that tubes have driven 600 ohms in the past is with an output transformer, but the transformer can be a barrier to transparency. Direct-coupling is the only way I know to get around this.