Hi BB,
Use Dolby Pro Logic II, Movie or Music modes. Some stereo recordings will sound wonderful in 5.1; others will benefit little or not at all. It depends on the original source recording, and how much out-of-phase ambient information is on the original stereo disc (CD or LP or tape).
The DPLII Movie mode may work better than the Music mode for some music. dts also has similar decoding modes for stereo, which are good but not quite as effective for some recordings as DPLII. If your receiver has Logic7 (it will only have it if it's an H/K, since Harman owns Lexicon), then try that too. It's frequently more realistic than DPLII for large classical, choral and opera works.
If you set your five-channel levels carefully and do a bit of tweaking with DPLII, it will not sound "mushy."
Regards,
