Tonearm Tracking Woes

I have what I feel is a prety good analog rig - a Teres 255 table, ZYX Airy 3x-sb low output MC cartridge and an Airtech MG-1 linear tracking air-bearing tonearm. Phono preamp is Supratek Syrah.

I've generally been happy with the results, but recently I find that I hear what can be described as a sibilince (sometimes even outright distortion) on some high notes. Also, when I use the HiFi News & Record Reveiw Test LP, I find that my tonearm/cartrige scores really badly on the tracking test tracks - I get noise on track 4 or thereabouts - well before the 'torture track'. I'd welcome advice on what may be amiss, how I might correct things or if I should really get a new tonearm.

I replaced an OL Silver tonearm with this Airtech and found the Airtech to perform better at the time. The Airtech ( is not really a known entity but it appears to be a poor-man's Air-Tangent tonearm. All told, it's definitely a great bang for the buck, but perhaps I need to go the extra mile and get a new tonearm (I don't have any 'surge tank' though - I don't even know what that is, or if that would be a partial solution). My primary choice would be a Schroeder Model 1 or DPS, or perhans a Model 2.

Anyway, I'd really value advice from you analog gurus on best next steps to optimize my rig. Thanks in advance for any advice.
I also have a TT with a linear tracking arm, although it's a vintage one. Perhaps you could lower the arm until the sibilance is at a minimum but you still have the detail and soundstage. I use a Grado gold, which easily mistracks if setup isn't perfect, and it tracks great on my TT.