It has been years since I last visited this forum but I need all helpful input I can get.
I have the opportunity to acquire used Ascend Sierras for my fronts. I currently have NHT Superones as fronts and center and Superzero surrounds.
Due to space constraints in my custom cabinet I don't have room for any speaker larger than my current Superone as a center channel. It is lying on it's side. Another Sierra is not an option.
Would this pairing be disaster -the Sierras with the NHTs?
No I don't think it would be a disaster, in fact, it might work nicely.
The Sierra-1s will be a noticeable improvement over the Superones, but because the Superones and Superzeros are reasonably neutral sounding speakers, a blend of those models, as you described, might be pretty decent. It won't please the perfectionists who insist that the front 3 speakers be an exact match, but I wouldn't worry about it.
You will find that the Sierra-1s are more sensitive than the Superones, they will play louder at the same volume setting. But you can adjust the individual channel volume controls to fix that.
What price are those Sierras going for?