The Spitfire II is the only legacy custom shop amp Diamond makes that has a Marshall style clean channel with breakup. The other custom shop amps i.e., Phantom, Nitro, Heretic, Hammersmith, etc. all have a more modern clean channel that stays clean longer. All of these are 2.5 channel amps.
For a single channel amp with 4 inputs, you can get a custom shop Balinese or Del Feugo though. Those are class A amps that will water your eyes with vintage syle Fender amp cleans. The Balinese is a monster slide amp for blues. Baltic Birch cab, 6L6's and GZ34 rectifier. Both of these amps are similar to Roy Blankenship amps. He used to be part of Diamond.
Call, PM, or email with any questions. I'll be glad to help.
I own a Diamond Spitfire (I) SN 100001 and it has a great clean that takes pedals very well. It reminds me Hiwatt amps. Roy Blankenship did work on the design of the Spitfire. This is a terrific amp. I think that Spitfire II shares the same channel 1 as the original Spitfire.