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.
1.I would immediately check 'professional setup' for everything: VTA, alighnment, VTF. It seems to me that VTF set up is too light. I'm sure I would find glitches to 'slap wrist' for:). It should be close to 2g on Shelter. In Shelters it's always better 'heavier' than 'lighter'. Bare in mind that professional set ups are very often done without testing and it's NOT the case with Shelter that needs lots of trial-errors to 'dial' the best VTF. Certainly it's time consuming and inconvenient to find a right 'dial' so I wouldn't count on professional help too much.

2.RB250 arm is raw(just like raw boat with no motor) by itself and needs to be modded:
Incognito rewire kit(I prefer it to even Origin Live), expressimo couterweight would bring it to the totally different level of performance.
at what frequency do you detect the peak?
Thanks for your responses. Sorry, forgot to mention peak is between approx 1.4-1.6k. The arm has been rewired with Cardas wire. I'm aware that lots of people replace the counterweight- was hoping to avoid spending more bucks but will def. consider a counterweight upgrade.
You can easily experiment with VTA to see of that is your problem. Adding platter mats is the same as lowering the arm. Even layers of posterboard etc. would suffice for this. I tried 3 leather coasters once and it actually improved the sound. Yes, mats will affect the sound, but right now you just want to see if this is the problem and I wouldn't be surprised if it was.