Wade,
I'm not surprised that the Advent cassette sounds different in its treble range from the Barclay-Crocker. Given the critical level matching required for proper Dolby-B playback, system output for those tapes was always hit-or-miss—sometimes brighter and sometimes duller than it should have been, depending on whether the particular playback deck in use had its "Dolby Level" set too high or too low, to say nothing of the proper azimuth alignment on those tiny heads.
Do you have any of the other Advent releases? There was a Horenstein M1, and I remember Horenstein's Nielsen 5 with particular fondness, especially the first movement, despite the one-measure-early side drum entrance by Alfred Dukes—otherwise the best realization of that drum solo I've ever heard, live or on record.
James