I really enjoy the i34 as well. Hell, one glance at my post history should give that away.
So far, it's the only affordable tube piece I've encountered in my 8 years of travels that performs with and in some
respects beyond super integrated amp's that weigh in the $8,000 USD + range.
Your concern about the i34 'not sounding equal' at all frequencies is interesting, as its a pretty damned linear (balanced sounding) integrated amp as far as tubes go. It could be the batch of JJ's you have, it could be the speakers, it could numerous things. That said,, I agree with Baumer... SED EL34's and Tung Sol EL34's offer a healthy leap in performance over the stock JJ's.
Personally, I enjoy the Tung Sol's simply because they make me want to sit down and listen to music. Darn near everything about them is better, to include the things that I value most in any piece of hi-fi; tone and realism. Hope this helps! Meanwhile, enjoy the amp!