The V1 is the closest to the side with the input. Only replace the tube if you are having noise issues. Some SPAX7 tubes, (and really any 12AX7/ECC83) can be low noise. It seems like it can be the luck of the draw. Looks like you have 4 older SPAX7 tubes so you could even swap them around to get the one with the lowest microphonics in the V1. They will have less effect on the V2, V3 and V4 position. If I remember right the V2 is only 1% as loud as V1, and V3 is only 1% as loud as V2, etc. So if you have a low noise tube in V1, you can usually get by with lesser grade tubes in the other positions.
The tube power amp will definitely give you a different sound and feel. Some people like using an SS power amp for their main sound but most only use them for their wet (effects) signal.
In case you didn't know already, the biggest complaint about the Studio Pre is getting the volume balanced between the rhythm and lead channels. Not sure if you will be using both but just thought you should know that if is giving you trouble you are not alone.
If you have any overdrive/distortion pedals don't run the drive too high on the Studio Pre's lead channel. Then you can use the overdrive/distortion pedals to make a noticeable increase in the drive that won't just turn it into unusable mush.
I have been running my Studio Pre into a tube power amp with a tube screamer and tc bld up before the Studio Pre input.
Keep rockin.
Bobby