The RME ADI-2 Pro fs R with the E1DA APU Preamp + notch gives a very high performance rig for 1kHz SINAD measurements up to approx. 10Vrms.
The APU costs around $100.
In mono mode, the RME Pro's DAC can reach around 120dB SINAD, and the ADC with the notch seems virtually unlimited. And the ADC is able to measure down to around -115dB THD+N without the APU (so at any frequency).
The APU preamp also helps for very low level measurements (like -60dB dynamic range).
The RME is also very good for a wide set of measurements, with its auto ranging functionality, and its input impedance, even in mono, is closer to a realistic load than the AP.
It also has a quite flat FR at 384kHz sampling rate.
Add a good attenuator (I now use the cheap but excellent $50 Behringer Monitor1) and a good True RMS Multimeter and you're ready to go.
For headphones amp measurements, the excellent $25 E1DA load is a no brainer.
Foresee to solder a pair of cables on it for ease of use.
The Virtins Multi Instrument software is so widely capable that you'll need time to get the best out of it. But its an incredibly capable $300 software, allowing .Net automation.
If you want to measure loudspeaker amps, you'll need loads and a serious attenuator able to take 50V or so. The maximum input voltage the RME allows is 24dBu (around 12Vrms).
I use the Linear Audio Autoranger II to attenuate down to that level without too high an impact on measurements.
But, to be fair, I'm not much into that.
Of course, this setup will not be a replacement for an AP for a professional application.
And, although the RME allows a much wider level range with high performance results than any other DAC and ADC I know of, it's still much less flexible than the AP.
But it will allow most of the measurements we see here with a good accuracy.
Just be aware that, with this kind of custom gig, you'll always have to question and double check the measurements results you get, to make sure your measurement hardware or method is not influencing too much your results.
But that should also be true with a $30000 Audioprecision.
And, as a hobbyist, this is also in the learnings that the fun resides.
It just takes time.