There's lots of good advice in this thread, the first bit you should heed is that you shouldn't spend any more money until you have done more comparative listening to what you have. I do have an issue with the comment:-
I would get peanuts for the arcam avr300 and would rather keep it.. From reading its meant to be amazing for stereo... . I'm debating selling the primare at the minute...
Here's my understanding of what is written about the Arcam AVR300 which dates from 2004. At the time was rated at one of the best AVRs (at around £1300) for stereo, apparently it was likened to the Arcam A65 (read reviews on this if you like) which retailed around £380, of course it goes louder as the A65 is rated at 40W a channel, not 120W a channel like the AVR300. Now you stated you have a Primare I22 which is apparently a class D intergrated amp (not a power amp.) which is rated at 80W a channel which you can buy new for around £1000 at the moment and which originally retailed higher (£1250). The I22 could also be purchased with an optional DAC, or this could be purchased and fitted later.
So IMO 'from what I've read' the best solution if you only want stereo is to sell the AVR300 and get a DAC for your I22 (if it hasn't got one already), only I wouldn't, I'd do what you've been advised to. Compare the Arcam when connected as advised by Lucid, so you find out which inputs you prefer, then compare to the Primare so find out which device you prefer. Your house, your speakers, your ears, you may prefer one over another regardless of what you've read. Only when you've done this can you make an informed decision.
You could choose which solution you prefer the sound of most, or could keep the AVR300 because you'd get peanuts for it if you sold it and because you've read it's meant to be amazing for stereo, then sell the Primare because you could get £550 in part-ex for something else. The choice is yours!
Out of interest, is the AVR300 actually amazing for stereo and how does it compare to the I22, especially seeing as there wasn't much difference in their original msrp's (at least the ones I could find).