speaker impedance in parallel

[beansprout]

I recently decided to experiment with my speaker configurations by hooking up my Paradigm Signature S8 in parallel with vintage Polk Audio RTA12's. I am unable to find the impedance of the Polks, but believe they are 6 ohm. The paradigms are 8 ohm and my Macintosh Amp has speaker taps rated at 8, 6, 4, and 2 ohms. I decided to go with the 4 ohm taps and if my calculations are correct the speakers would represent a 3.4 ohm load. Is it wiser to go don to the 2 ohm taps for safety or would 4 ohm at the amplifier be ok?
If I go to the lower 2 ohm tap and the impedance is actually 4 ohms, what would I be giving up? More db's?
Thanks in advance.

 

[kfalls]

The Polks are 4ohm. The parallel calculation is 2.666ohms. I think the 2ohm tap would be better suited.

 

[toxcrusadr]

Yikes!

 

[StimpyWan]

I hope the Mac amp has autoformers...?