Here are the Blaupunkt SWC Ohm values fo the Blaupunkt Stockholm 400 and the Bremen SQR 46 from Blaupunkt Germany via E-Mail:

Button TOLERANCE
Volume - from 18,90 to 29,4 KΩ
Volume + from 13,25 to 18,85 KΩ
Seek - from 9,20 to 13,18 KΩ
Seek + from 6,60 to 9,18 KΩ
SRC from 420 Ω to 2,2 KΩ
MUTE from 2,3 to 4,68 KΩ

Blaupunkt wrote that these Values are similar or close to Pioneer.

 

I have an older Pioneer unit that came with my S2k and most of the audio controls work out of the box.

If the Blaupunkt has similar values to the Pioneer, I'd imagine it would work with the OEM stereo controls? Or am I missing something?

 

Yes, you are missing something. Honda OEM uses it´s own Ohm Values, wich the OEM Honda Pioneer radio matches. These Ohm Values differ greatly from Pioneer / Blaupunkt aftermarket Head Units. Therefore, you need a DCI for aftermarket Radios.