Two crossovers at the same point will essentially create a dip in response around that point - the two crossovers' slopes will basically combine, exaggerating the effect of the slope. It is something that is generally recommended against, but with the SMS-1's ability to show you exactly what is happening I'm a little less leery of the idea (after all, you can see the flat response, so there's no dip created).

The one problem I see with using the SMS-1's crossover at all when also using a receiver's crossover is that any LFE soundtrack data (the information contained only in the .1 channel) above 80Hz (or wherever the SMS-1's crossover is set) will be lost. The ideal approach would be to turn off the SMS-1's crossover since your receiver already takes care of bass management. If there is a peak at 80Hz afterward, you can try to flatten it out with the EQ. You might also play with your receiver's crossover point if it allows it. Jongaro did this - he moved his crossover from 80Hz to 60Hz - and was able to get a flatter response, but not all receivers have enough bass management flexibility to make this a convenient option.