I had been using Spectron monoblocks full range for a couple of years. When I had an opportunity to get the Double Kronzillas (two output transformers on each monoblock), I couldn't resist. The Krons do some amazing things, but the Spectrons shine on the bass with their grip and definition on the woofers. In monoblock mode, the Spectrons have a 100k bandwidth, so they have the speed to seamlessly blend with the Krons.
I would not recommend touching the internal crossovers nor adding an external crossover. I suppose I could pick up a bit more headroom by putting a high pass filter in front of the Krons, but so far I've never run out of volume and why add more stuff in the signal path? On the VR5-Annis, the Spectrons need to be attenuated by about 3 to 4 dB when both amps are driven by my ARC REF3-- there are lots of options out there for attenuators. The net impedance of the amps is lower than the REF3 published specs recommend, but I've not noticed any audible rolloff of the highs.
I tried the biamping route when I auditioned the Krons just to see if it was worth keeping the Spectrons instead of taking the price hit on the used market. Well, I was amazed how well they worked together and have never looked back. The result is definitely more than the sum of the parts. By the way, my friend is using VR5-Annis also. I don't know if he's going to keep the Spectrons or go full range with the VAC 300.1a.