There are only five preamp tubes in the standard recto series of amps, not six. If you have a rectoverb or tremoverb, there may be six tube, I've never opened one up to know. So if this all doesn't apply, you'll need to be more specific about your amp.
Starting nearest to the input and moving toward the power transformer you have:
V4 FX Loop
V5 Phase Inverter
On page 4 there's a functional diagram of the amp which will tell you which triode of each tube does what. http://schematicheaven.com/boogieamp..._solo_head.pdf
The clean sound on the rectos is regarded as being pretty bad. You can try an AT7, but I don't think it will clean it up the way you want it to. You'll get less gain, but it'll still be dark and kinda dull. I'd change V1 to an AT7 for the most prevelant results, or V2 for minor results. Both changes will effect your gain channels though.