The best upgrade would be to change the cartridge to a brand new one with microline stylus ... like the Audio Technica vm95ml.
It's not an expensive investment (150 USD aprox).
Other good action in the vinyl world is to have clean records (I mean wet cleaning with distilled water / Tergikleen surfactant) ... the "upgrade" can be really great.
Now I'm showing my age here - to
@DuncanTodd too and apologies for late reply.
The 881S was an excellent cartridge and the Stereohedron stylus excellent at its job. At todays pricing, I'd suggest the 881S must be the thick end of $500 if made today. Memory has failed me as to what exactly the Stereohedron tip profile was, but I'd put a 881S body with Vividline or Jico Shibata replacement styli rather above the internally crude but well-tipped VM95ML and more like the excellent priced VMX700 series models AT now make, with nice metal mounts and cheaper pricing for the 740 and 750. I'd put the diamond before the cantilever here, so a VMX750 perhaps.
-There was/is a YouTube vid of three AT OC9s compared. same record and tonarm, each properly set up and recorded as best as possible. the elliptical stylus sounded fine initially and very 'vinyl-like' in a slightly band-limited kind of way. the ML stylus had far more of an 'etched sparlke' up top in the pickup design, which I like, but which could maybe be called 'one note sparkle' perhaps (just a subjective vibe that wouldn't bother me at all). the OC9 with SH stylus was to me sublime, keeping all the 'detail' the ML gave, but presenting it in a more subtle way (I can't explain it, but it was less 'one-note' to me and quite delightful)
If an original SL1200 with 'dome' in the lid over the arm pivot area, I'd suggest taking the lid off when playing (the deck used to be terribly feedback prone) and site the deck very carefully to minimise vibration ingress - my school-pal had one and you could almost play tunes into the speakers if the plinth was tapped while playing - my SL110/SME was exactly the same, only I didn't know much then, having just started out in the industry). The 1200mk2 had extra damping under the platter and I believe, more damping inside the deck's internal cavity itself from memory. I was also able to get stellar performance from an SL1500 with siting, lid removal when playing and to get the arm level, an additional cork mat on top of the heavy Technics original, which isn't so far removed from the more costly 1200m1, so good luck.
Back to the X5 itself. AT VM pickups have slightly lower output at 5cm/s than the usual 5mV (ATs are usually 3mV I believe), so the hf headroom may not be an issue at all. I repeat that the lack of a subsonic filter for such a low priced box I'd say is a major issue though, because as said above, low priced turntables not properly sited are going to have all manner of things going on low down and especially below 20Hz. Since on most commercial vinyl albums, there's pretty much nothing useful below 40hz or so, a filter working below 25Hz would be an asset and not a hindrance here, even if it wasn't switchable as it might be on a more 'sophisticated' phono stage.
