I think it would be MOST useful if users were able to select/change a custom 30-second range of the preview window.
I play short punk rock songs and the current setup doesn’t allow me to preview crucial sections of my songs. 
Digging the online mastering functions otherwise. Radio DJs have noted that my masters are better sounding than ever
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I sent some very similar feedback to them myself a couple of weeks back. Here’s hoping they feel the same way.