You didn’t mention budget. I’m sure the Benchmark stack would be better than the IDA-8. I had the latter, and while it offers a lot good for the price, you would notice the difference on your Harbeths. I have a pair of P3ESR, and to me, the IDA-8 used with them sounded fine, but not as good as more expensive gear. (I'm using a Classe preamp and Marsh amplifier.) The IDA-8 digitizes everything, so you will get its signature even if you use your own DAC.
I wish I could suggest a compact integrated up to the standard of your Harbeths, but I am not familiar with that market. If you buy the IDA-8 new, I’d make sure you can return it if you don’t like it.
Active or powered speakers would help solve the space dilemma, but IMO Harbeths are hard to replace for natural and non-fatiguing sound. So be careful . . . there are worse things than a crowded desk. If you like the Naim otherwise, make space for it if you can!