Trumpet archons are surpringly similar to planetars. They're under-CR'ed celestials with innate cleric casting, with the planetar being the obviously stronger one. Trumpet Archons have a preset LA of +8, but that would put it six levels behind a single-classed cleric with no way to catch up.
For some weird reason, trumpet archons lack their lesser brethren's Aid spell-like ability. I know they can cast it as a cleric spell if necessary, but it's kind of weird a lantern archon is better at granting people temporary HP than the far more powerful trumpet archon. The spell-likes it does have are getting obsolete by now, though.
The ability granted by their trumpet is interesting and can potentially stunlock an entire encounter, but the saving throw DC is low and I find myself wondering why one doesn't just cast a few deliberating spells. Similarly, a +4 greatsword isn't that useful if you can just buy one (with actually useful enchantments instead of a flat +4 bonus) for one-fifth of your WBL
All things considered, the trumpet archon is a celestial powerful because of its cleric casting and racial abilities, but lacking the planetar's big guns. A +4 LA seems most appropriate, though I'm considering +3 LA. As always, your opinion is valued!