It really is hard to get, or give, a straight answer...It really is personal taste and you'll have to try different tubes to find the sound you want...I've found that 2 of the same tubes of the same brand will sound different. Some will provide higher gain than others of the same make. There's usually not a huge difference in tone, but different tubes will provide subtle differences...

I recommend Tung-Sol pre-amp tubes....they provide high gain without being microphonic....

Probably no need to replace all the pre-amp tubes. Just try replacing the first 2 closest to the power tubes...those are the pre-amp tubes that provide the overdrive...

Let us know what happens if you decide to try this...