I've seen them live 5 times, first in 1973, twice in 1975, 1993, and 2002. The 1993 show was a bit of a shock, as Ian's vocals were cracking all over the place, something was obviously wrong with his voice. They piped in a recording of the opening vocal harmonies to "Songs From The Wood" while Ian mimicked singing it ... he wasn't, and it was clear that he wasn't! When the actual singing came in, his voice was weak and cracking. Turns out he was suffering from some sort of throat disorder at the time, which was speculated on all over the (then) early Internet what it might be, but was never really clearly explained (Ian has always been protective of his personal life).
In 2002, I had 3rd row seats to a Tull show, so was up close and could hear what was going on on-stage rather than thru the PA system. Ian's voice was much stronger at this show, although it still had a bit of weakness in it, probably due to age and the aforementioned malady from 10 years earlier.
If you listen to the early Tull albums, starting with "This Was", you'd notice that Ian never really had a strong voice to begin with ... it had a nice bold Baritone to it, but started to lose power in the Tenor range. What Ian did to compensate for that was to double-track his vocals on the recordings, closely following the first track but once in a while being slightly out of sync. It helped to 'thicken' his voice.
Live shows it didn't really matter, since Ian is such a huge presence onstage with his stage antics and magnificent Flute playing, always backed with a strong Rock n Rool band, although sometimes Tull would tour as an almost acoustic group, which I suspect might have been the show you just saw, hence no "Aqualung"
There have been several 'flavors' of Jethro Tull over the decades, not just the Rock n Rool one from the early 70's that we all know and love, but the Folk, Jazz, and Blues versions of Tull which still holds a hard core fan base in both the UK and US.
Sorry to get long winded in this post, but thought a little background explanation on what you experienced at your Tull concert was in order. Hope it made sense :)
Cheers,
MM
Not at all, thanks for the good info!