I'm not sure how different kt-66's are from kt-77's, but when i bought my rectoverb it came with the 66's. My impression of them was way too bassy for the amp. They didnt seem to have much dynamics, it was just too much juice on the tone. Kinda like it had a boost on it all the time, but not in a positive way. More like in the way bass can over power the rest of the tone.
Again, these may have been old, but i didnt care for them.