Hi all. I wanted to see what everyone's opinions are on changing preamp tubes. I've got a Hartke 3500, and will be recieving a SWR Grandprix. I'll possibly also buy a ampeg SVP Pro. I've also been doing some research for myself and friends on tube preamps. It's suggested to replace 12AX7's with 5751's. I can see where they are coming from, with the 5751's being a bit better constructed, less microharmonics..etc. All the research I've done points to Sylvania JHS/CCA ones. I was thinkin about buyin a few of each brand, testing them out in both rigs. So my question. Anyone ever retubed a Hartke, SWR GP, SVP Pro (What tubes come in the SVP Pro anyway?) And had some success at it? Any recommendations? I've also never yanked and replaced a tube, but it can't be too hard. Any suggestions/cautions? Thanks, and I know it's all personally subjective. Everyone's ears are different. I'm talking a distinct advantage, if that's even possible.