The reason you have to raise the volume much higher on the classical works you mentioned than on pop recordings is that the classical recordings have much wider dynamic range than the pop recordings. Dynamic range in this context refers to the difference in volume between the loudest notes and the softest notes. Most pop recordings are compressed to very narrow dynamic ranges, while many classical recordings, especially of symphonic works, have far greater dynamic range. Our hearing mechanisms tend to sense volume based on the average level, rather than peak levels (especially if the peaks are brief, as they are on many classical works). So for a given setting of the volume control a recording having limited dynamic range will sound louder than one having wide dynamic range, since a recording having limited dynamic range will have an average level that is closer to the peak level than in the case of a recording having wide dynamic range.
I’m not sure, though, why the sound starts to get muffled above the 65 setting, or why you don’t get much "oomph" on the symphonic works. Unfortunately I haven’t been able to find an impedance curve for the speaker, showing how its impedance varies as a function of frequency, which might have been helpful in addressing this question. FWIW, though, I did find one thing that might point in the direction of the speaker as being the culprit, and another thing that might point in the direction of the amplifier.
Re the speaker, from this review:
We’re not going to be too critical of the need for careful system-matching, but we do have to mark the C2s down for a bass performance that’s just a touch vague. Tracks that rely on deep thumps for drive reveal a slight lack of bite and tautness in the low-levels.
Re the amp, I see that its 8 ohm power rating of 100 watts is identical to its power rating for 4 ohms, which is not encouraging in the case of a solid state amplifier. Although it’s hard to extrapolate useful information from that fact without an impedance curve for the speaker.
In any event, good luck as you proceed. Regards,