Try ModWright preamps - they are certainly not warm and stereotypically tubey. Dodd preamps also. Both these manufacturers use 6H30s, which might represent the sound you're after.
Whatever you choose, using balanced outputs has always worked best IME with both the X150.5 and XA30.5.
Whatever you choose, using balanced outputs has always worked best IME with both the X150.5 and XA30.5.