I don't have much experience with any recent vintage Rotel equipment, but there are a couple of potential issues here.
1. "1/3 on the volume knob" is not a reliable indicator of anything. If the source is pumping a fairly high line level voltage into the amp, a 1/3 setting could be asking the amp to deliver near it's maximum power. A better question would be what type of dB volume in the room are you hearing when you experience this clipping? If you are hearing clipping at 85 or 90 dB, then there is a problem somewhere. Either some component is defective or misconfigured. If you are listening in excess of 100 dB, you may well have nothing more than a power issue. Radio Shack sells an inexpensive sound meter that will help you determine this.
2. That said, some amps can have trouble delivering their rated power at lower impedances. The Spendor S8 has a nominal impedance of 8 ohms, but a minimum of 5 ohms at some frequencies. I wouldn't think that should be a problem for a reasonably well designed amp, but some do have power and stability problems as the speaker impedance drops.
The formula for wattage is voltage times current. However, on a transistor amp, when the impedance drops from 8 ohms to 4 ohms, the speaker is asking the amp to deliver the same voltage, but twice the current. Some amps aren't designed to operate well at lower impedances.
Anyway, I'd think the first issue is to determine just how loud you are really playing the stereo. Once that benchmark is set, you'll have a better idea if your amp is misbehaving or you just need more power or a different amp.