I operate the 536 and the HP2 with SpectrumForce mag mount mobile antennas and see no difference in reception using those. Both are excellent for the price.
The 536 with the telescope antenna it came with is disappointing. I can say the same for my old 890 and models I had before that. I haven't even tried the rubber duck on the HP2, for the sole reason I am using that in my office and I have the mobile antenna affixed on the side of the steel building I work in.
Taste in sound is personal nature. I do like the crisp sound of the HP2, but I didn't think the 536 sounded muffled. Bassed? I can hear that. Since I put the BC15 on the unit (rear port, not front) I find that it's a bit sharper.
I am sure there is a logical explanation for design of the speaker on the bottom of the 536 (less dust to impact sound quality?) but I haven't been a fan of that design.
Anytime I buy a scanner, I look for an upgraded antenna and external speaker, because the OE tends to be a let down in those two all important areas.