A Unity 500 for £350? Bargain!
Does it have an Effects Loop?
Get the Unity & plug the Darkglass into the FX Return, bypassing any "colouring" that the amp's preamp may bring, though there may be some colouring from the power amp & speakers.
Have been having a similar discussion for quite a few years now, but due to not being able to stretch to one of the "top notch" FRFR / PA Powered cabs, I've been using my Ashdown MAG 300 combo with a succession of Multi-FX/Modellers (currently a Line6 POD Go). Works for me.