Your salesman is an idiot. Tell him I said that. 
The nav unit operates pretty much independent of the audio system. Wether or not you have DSP or HK or whatever the hell stock stereo you have does not determine how the nav functions, nor vice versa. He is RIGHT that the 3 series does not have DSP but ALL, I repeat, ALL BMW stereos have bass and treble settings.
Tell him to do some [Oops!] research on the cars he's trying to sell.
The nav unit operates pretty much independent of the audio system. Wether or not you have DSP or HK or whatever the hell stock stereo you have does not determine how the nav functions, nor vice versa. He is RIGHT that the 3 series does not have DSP but ALL, I repeat, ALL BMW stereos have bass and treble settings.
Tell him to do some [Oops!] research on the cars he's trying to sell.