Help with turntable response anomaly

Hello all,

I've just replaced my Pro-ject 6.9/ Shelter 501 Mk 11 setup with a Nottingham Analogue Horizon/Rega RB250 (retained the Shelter). In fact all electronics (catridge, Lehmann Black Cube phono preamp, power amp, cabling) are identical to previous setup. But there is an annoying peak (narrow cue, maybe 1.5-2 db)
that really makes listening difficult. Not imagining it, I've spent 30+ yrs tuning large scale sound reinforcement systems around the world. This is my first TT without an integral arm, so I had it set up by pros, who've clearly done VTA etc by the book. Problem is obviously mechanical, as all electronics are same
as previous setup, only TT & arm have changed.
Any ideas? Being an RB250, VTA is not immediately adjustable. Is that likely to make a difference to the response? Tried changing cartridge loading, no go. TT is on a wall mounted aluminium slab, & is level. I don't have enough experience with this stuff, would value your input.