Harley Benton PA-100 Power Attenuator – Thomann UK
That will serve as load box and deliver a line output to the AI. HB make decent kit but not soooper stuff so that load/power soak has its limitations but I doubt you will find better cheaper.

Main limitation is that it is not really up to a 100W valve amp, 30 watts, 50 at a pinch.

The mixer COULD be useful. That line output is 'raw' that is it is devoid of any filtering or tonal effects from the speaker. Top end loads incorporate "Speaker Emulation". This is a tone shaping circuit. The mixer has EQ and with a bit of upper bass lift, mid depression and rolling off the HF past 8kHz or so, you remove much of the 'fizz' of raw guitar sound.

As stated, the make and model of the amp would be delightful!

BTW don't run the ***t off even a transistor amp without a suitably rated load.