I agree with upgrading the source first. But, don't discount the importance of a good PC on the preamp. The preamp influences the timbre of your system, especially a tube preamp since tube rolling can change detail, soundstage, dynamics and colour.
Finding the right PC for your preamp can influence the sonics dramatically. One thing to keep in mind is that the preamp is a low-current draw component and many aftermarket PC's have heavy copper windings more suited for an amp, so
Since you are thinking of upgrading the power stage, I'll second the recommendation of adding a dedicated AC line (if possible). This line will provide you with cleaner, more stable current, since you will now be isolated from the rest of the devices in your home.
Finding the right PC for your preamp can influence the sonics dramatically. One thing to keep in mind is that the preamp is a low-current draw component and many aftermarket PC's have heavy copper windings more suited for an amp, so
Since you are thinking of upgrading the power stage, I'll second the recommendation of adding a dedicated AC line (if possible). This line will provide you with cleaner, more stable current, since you will now be isolated from the rest of the devices in your home.