Is it my amp or my speakers?

ignoring your speakers for a minute, i see the other gear you have i would say the sound is bright, a little harsh in the treble, a little recessed in the mids, strong in the bass

so if you seek brightness, it has gotta be the speakers - i don’t know the tektons, much less your tektons... they make so many models covering the waterfront, no idea what is the sound character of yours... tekton’s better speakers certainly are known for their impact, neutrality and clarity - your specific model, maybe not... other poster below says they are smeared/smoothed over ... that may well be spot on...

Assuming all units of model X sound the same is not a safe assumption. Some may operate better than others. Especially older gear and even if work was done on it.
Not saying it’s the amp but not possible to rule that out with the limited facts on hand.
If not happy then try something different and see what happens. The Bel Canto c5i is an integrated that is never inherently bright and can drive most any speaker to modest volume well and would not cost much to try used.

But assuming the amp is working well I like that Krell very much and would hesitate to part with it if it has served me well over the years.

The Tekton Lore would be more special were you using a modest powered tube amp. With the Krell in good operating condition or with other similar quality beefier SS amps, you have unlimited speaker choices that will match up well.

Thanks all for the v. insightful analysis. Got what I had hoped for from the community. Majority verdict: the speakers. I like the Tekton business model, but the array of speakers is way overwhelming (there were many fewer options when I purchased mine, 8-1/2 years ago). Back to the endless task that is research! Thanks to @smrex13 for the very good suggestion of "Places like Crutchfield and Music Direct have trial periods that will allow you to hear the speakers at home with your system. "

New speaker time

Add Audio Advisor to that list; some independent B&W shops also allow for at home auditions (e.g. Audio Connection NJ).

These speakers will sound best when you are on the woofer axis.  If you are far above it you'll lose quite a bit of treble.

Assuming your seat/speaker are aligned if you still have issues,sit 2’ from the speakers. If they still sound colorless, it’s the speaker, otherwise it’s the room. :)