I have an Audio Precision System 1 with Duke's (Audio1Man in GroupDIY) USB interface and it works great, but its impractical to move around, so I finally got this
https://quantasylum.com/collections/frontpage/products/qa401-audio-analyzer for a bit more than a 2 channel common USB audio interface and I couldn't be happier, the thing can read down to 0.0004% THD which is around the same as my AP SYS-1, the noise floor is at -113dBV. It can measure noise floor, frequency response, THD, THD+N, IMD, speaker impedance and more, and it is USB powered. It uses a proprietary software which I think looks and works great, and you don't have to worry about calibrating anything. It is not perfect, bandwidth is limited to 80KHz so if you want to measure THD at 20KHz then you are not going to see many harmonics, but still, for the price it can't be beat, technollogy these days, huh?
BTW I saw the other day in an audio magazine (perhaps its old news) that AP is selling the software alone so you can use whatever hardware you want, but I think the software is over $3000

thats a bit too much just to get the "AP" logo on your measurements, but for a small audio company who wants to publish their measurements and be backed up by the AP name, then its a good alternative, not so much for DIY thou.