>Almost all of the bands (even the Stones) don't sound anything like they did >at their prime.
Eh? I think you must be confusing live v recorded and either have never heard these bands live "at their prime" or are remembering the atmosphere rather than the sound?
Most rock bands sound dire live. Mostly because there are very few good singers around but just as much because the musicianship isn't particularly great either. There are exceptions, but not many.
Amongst those bands, Led Zeppelin are particular because they are markedly poor live [so much so that it's extremely ironic that Page should moan about the quality of bootlegs and Page / Plant at the quality of their Live Aid performance - which, admittedly took mediocrity to another level, but it wasn't that far of their par]
Live aid was poor mainly because the 2nd best musician in Led Zep had died years before, and the one musician they do have didn't turn up.
In the studio they were brilliant though, but they'd easily be better today, simply because technology has improved.
My 8 year old was introduced to Led Zepp from the clip in School of Rock, but I've played him lots of live clips and studio and he agrees they aren't worth seeing. £1250 to go back and see them at Nebworth in 1979 would be good value, but £125 is laughable to watch them today. Paulo Nutini'll be worth watching, but you can do that a lot cheaper.
Today, the attraction for getting 2 tickets is the inflated price they'll resell for. I'm thinking it might even stretch to one of those new Panasonic 1080p plasmas, and perhaps a few Led Zepp original CDs with the change.
Go to watch them for the atmosphere by all means, but they never had a "prime" for live material.