Although it isn't extremely significant I have some questions that might help diagnose the problem. Where is the volume control located when this happens? For instance, is the flat part of the knob around the 3 o'clock position or more while playing for more than say 10 minutes? Also, what type of power supply are you using in regards to Voltage and Amperage?
The reason I asked about the volume control setting and power supply is based on generalities. In other words, just because the volume control is half way it does not mean that the amplifier is outputting half the wattage that it is capable of. Furthermore, I suspect that the power supply has an influence on how much the power chips are stressed while trying to provide the voltage and current that you want supplied to provide the SPL at your listening positioning that your speakers are capable of delivering. For instance, if my speakers are capable of producing 100db from 50hz to 20khz with 1 watt and yours are only capable of 86db measured at only 1khz then I will need much less power to achieve concert or studio levels at the same distance from the speakers. Unless, you are using line arrays and are theoretically in the near field rather than far field.
Anyway, my point is that Jason may need more info. For instance, how far are you from the speakers when listening, what is their efficiency, and how loud are you listening when this happens. The reason I ask these questions is because if you listen from 20' away and I from 10' from speakers with different efficiency then it can make a huge difference in the demands on amplifier. Furthermore, source might make a difference. For instance, I find myself cranking the volume on SACD's compared to newer CD's and DVD-A's. However, I'm guessing that having the volume control at a certain position does not determine the demand on the amplifier if the incoming signals are at different levels.
With all of that, Jason mentioned the vacuum tube circuit. It may be a good idea to replace the vacuum tube. From personal experience as a guitar player, preamp tubes do tend to become microphonic. Actually, some are shipped microphonic. I'm not sure how much it would affect these amplifiers since it doesn't seem to be used as a gain stage or under a great deal of clipping but vibration does affect vacuum tubes over time. For instance, strike a light bulb while it is lit. Although, tubes are much more robust they have filaments and when they get hot they are subject to damage from vibration. If the amplifier was used on a desktop with studio monitor/desktop speaker with significant volume then it could be causing some issues with the tube it's self. Preamp 12ax7 tubes are cheap enough to try a different one. Of course, you can spend a great deal on an NOS but I do not really see the point. Then again, you could just remove the tube buffer from the circuit and see how that sounds. From my experience vacuum tubes are great at harmonic/pleasant distortion as long as a diode is not between the pre amp and power amp stage. However, they are noisy devices from a technical standpoint. As much as I love vacuum tubes and amplifiers that use them I have not used them in my stereo system.
Hope some of this info is helpful. I'm not an expert by any means but I have accumulated a little knowledge over the years. I assume you are a music lover like me and want it to sound realistic or you wouldn't have bought the Sensation. Therefore, if I said something that you already know then I promise it is not my intent to be demeaning, I'm just throwing stuff out there that many people either forget to never learned.