The JJ's cound really nice in the amp. I have the LSS 112 combo and just retubed the whole amp with JJ's. I went with a matched quad of Groove Tubes EL84S (which are in fact JJ's) with a 4 hardness. The local GC had them and was good to me on price, otherwise I would have ordered from Eurotubes.
I put high gain ECC83s' in V1, V2, regulars in V3, V4, and a matched PI in V5. Made a world of improvement from my perspective. Mesa had "gray" tubes in there, which I think break up a little later than the GT 4's I put in. The darker pre-amp color of the JJ's took some of the high-end down a bit and I think the amp sounds better. YMMV.
As for the warranty, it's 5 year transferable, so it has to still be under warranty.