When converting from stereo to quad, the rear speaker data should be identical to the front speaker data, so it's a bit strange if the rears sound louder. Is it that XMPlay's volume control is only affecting the front speakers? If so, you could try the DirectSound or WASAPI output plugins, as each output system has its own volume control methods, which perhaps will work slightly differently. Another option may be to adjust the speaker balance in Windows' Sound control panel?
