Sound Valves VTP 101 Preamp Tube Condition?

How do I know when it's time to change the tubes in a used Sound Valves VTP 101 preamp? I never heard one before so I have no basis of comparison. I just picked one up from an estate sale and love the sound compared to my Forte 44, but sometimes I think I hear it distort when I push it. I'm not that into loud music and the only reason it gets pushed at all is because I have a low output Grado Sonata1 cartridge (0.5mv) that needs a little more gain to hear it sing. I'm using a Grado PH-1 phono stage rather than the built in phono stage because I thought it sounded better. I read in a previous thread that the VTP 101 responds well to choice tubes and would appreciate any suggestions. It currently has Sovtek tubes installed, age and condition unknown. Also, I didn't see any way to set bias on the board. Is that something only done for power amps or is there a circuit in the VTP that regulates the bias? Must the tube sets be matched? Do other preamps have provisions to set bias? If anyone has a circuit diagram and/or knows where I can get the specs on this preamp I would be grateful. It came with no owners manual. Currently I am using an Aragon 2004 but may repair a Golden Tube Audio SE-40 and/or a Dynaco ST-70(also available very cheap from the estate sale) if either will match well with the VTP 101, otherwise, they wouldn't be worth the bother.