Your speakers are rated at a nominal impedance of 6ohms but what matters to your tube amp is the real life impedance swings during varying frequencies. If your speakers offer a relatively benign 6 ohms and your amp has a 4 ohm tap, you should be alright. As other posters have mentioned, if your speakers present too much of a load to the amp, dynamics will be affected along with a flat uninvolving sound. I love 845 based amps, but they do require more attention to speaker compatibility.
