Love the look of those RCA's! And your sonic description of the L63's parallels mine pretty closely. Not my go-to tube(s) for rock or metal, but for classical they are an absolute delight. They partner nicely with a Cetron 7236 and a Russian 6N13S (6AS7G), are pretty decent with the TS 5998, but not the best synergy with RCA or Sylvania 6AS7's or 6080's being a bit too warm and lacking ultimate bass impact with those tubes. When it comes to the soundstaging thing, I popped the good old Frankentube (Sylvania 7N7) into the Darkvoice the other day and was once again immediately struck with the depth, separation and delineation that tube provides in that area. It was one of my top 2 tubes in the Schiit Lyr 3, but it failed to impress when I first plugged it into the (OTL) Incubus amp. Hadn't tried it in the DV up to that point, but here as well and partnered with the TS 5998 it does that soundstage thing big time. Beyond that, it's kind of a jack of all trades tube -- does pretty well at everything, but isn't an expert at anything.
And when I'm in a heavy metal mood, these cheapie Pinnacles continue to impress. They're a Russian 6S2S, but I have no idea of their vintage or what factory they came out of and they need at least 25 hours of break-in before some initial stridency fades away. Probably on the solid state-ish side of things and NOT to be used with the 7236, but with the TS 5998 and/or Chatham 6AS7G, they hammer out bass notes like none other. Definitely the go-to tube if you want to blow the drivers out of your 'phones at some point.