Thanks for the replies guys. Much appreciated.
My TX only has power, PPL unlock, PPL locked and Drive Power LEDs, no meter of any description. It had a stereo coder inbuilt, but it was disconnected in favour of MPX which works fine.
https://www.aareff.com/en/fm-transmitter/1-watt/ (1W is the maximum allowable for LPFM in NZ)
I've seen the video Hans put out, but the meter he used I can't find, borrow or beg from anyone locally. C19 has meant a redirection of finances away from the station (it's just me running it) so to buy one is out of the question, at least for the foreseeable future. Same goes for MicroMPX, sadly.
As for sample rates, the playout PC transfers audio over the network to the TX PC at 48k/ogg. From there, the TX makes it 192k to allow for RDS.
It sounds like it's probably more to do with the TX, or calibration of, and maybe a better lead between the TX PC and the DAC (a Scarlett 2i2, used as a sound card for the PC) might also help. It's just a cheap RCA to RCA with a RCA-6.5mm tip at the DAC.
Hopefully things will improve financially soon and I'll be able to buy a better TX and the gear to calibrate it, but until then we're not on air and I can just listen over the network or while I'm out and about in the meantime.