| I've never played the BEF Pro or the Xero Deluxe, but I found the Ultron to be kind of cold and clinical. Guyatone says both the Ultron and Ultrem have an all-analog signal path with digital controls, but the Ultron didn't sound very warm to me. Also, like so many other pretenders, they compare the Ultron to the Mu-Tron III. IMO, the Ultron has nowhere near the grit, grease and character of the Mu-Tron as a straight envelope filter. The Ultron has modulation choices that set it apart from the Mu-Tron, but it just didn't do it for me. I've never tried the Ultrem. YMMV. | 
