You want a nominal 82dB, which is 7dB less than the speaker sensitivity, so a nominal 0.2 watts will be about right. (Instantaneous peaks will be much greater than that, maybe 20 watts with an audiophile CD, which is why you have a powerful amp). The amp puts out 195 watts with 1.28v input, which is about 10oo times the nominal needed, so the voltage needed is the square root of 1000 times 1.28, or 0.040 volts.
Seduction has a gain of 100 which is 40dB, if you have the C4S boards. That means a nominal output of 0.40 volts if your cartridge is 4mV nominal. The instantaneous peak is about 2v for well-recorded vinyl (14dB peak to nominal).
Let's assume that in normal operation you are using the Quickie turned down 12dB from the maximum, so there is some room for quiet recordings, or parties, or digging into a quiet passage. That gives a net gain of unity, so the Quickie output is also 0.4v nominal. Do, you need a 1:10 voltage attenuator, or 20dB, at the power amp.
The first choice is to put the attenuator inside the power amp. It has a 20K input impedance, so a 180K resistor to replace the wire that attaches to the RCA jack will do the job. Very simple, unless the RCA jack is soldered directly to a PC board - then it might get a bit tricky.
If you want to leave the Yamaha unmodified, a reasonable second choice is to put the attenuator in the Quickie. This requires that you keep the resistances lower, but a voltage divider of 18K + 2K will do the job and have an output impedance of less than 2K, which is half the impedance of a quickie with PJCCS. You could easily go twice that impedance.
What I would do, myself, would be to get a pair of potentiometers, 20K to 50K, and install them inside the Quickie. You can probably glue them to the chassis next to the output jacks. That way you can adjust the gain in each channel, allowing for tube gain differences, and experiment to find the attenuation that works best in your system. The PEC pots are relatively inexpensive and quite nice sounding. You can use a smaller value, say 5K, with a series resistor so it adds up to at least 20K - that way the adjustment is less tricky and you can easily use linear pots.
