+1 what @kofibaffour mentioned at the end.
You should be ok. The Heresy's rating is 100 watts continuous and 400 watts peak. Most of the time you are using <10 watts max at normal listening levels. It would be difficult to run >100 watts continuous into a speaker. Just don't push the volume to 10 or 11, which would probably push the amp into clipping and which would damage both amp and speaker.
Question, and maybe just to be nitpicky... I'm assuming you are using 2 amps. Are you bi-amping the speakers (i.e. removing the jumper from the speaker terminals and using 2 amplification channels on each speaker) or just bridging each amp and using them as monoblocks? If you are bi-amping, then it is still only sending 100 watts/8 ohms to each driver. The 300 watts/8 ohms is the rating is for using the amps in "Bridged" mode as monoblocks.
-Jeff