I have a Marantz device (M-CR510) that I hooked up to a Polk sub using that exact method without issue. You are fine.

In fact, if you don't use the speaker level inputs on that sub, the low pass crossover on the back of the sub is bypassed which means that it would output its full range overlapping with your Diamonds heavily. PM6004 doesn't have an internal crossover like a lot of receivers do so this really isn't just a safe method, but the right method for you to connect the sub.

IMO, speaker level inputs are the best way to connect a subwoofer in general as the subwoofer and speakers then are getting the identical signal.

mattgomez wrote:

I have a solid state amplifier (Marantz PM6004) that is rated 60wpc at 4 ohms or 45wpc at 8 ohms.

I got a ridiculous deal on an inexpensive subwoofer (an old Polk PSW450). It's 4 ohms.

I currently have a pair of Wharfedale Diamond 10.1s, which are listed at 6 ohms.

Any guidance?

First of all, the subwoofer is NOT a 4 ohm subwoofer. It is a POWERED subwoofer, which means that its line in jacks have an impedance of 1000 ohms or more. If the speaker in the subwoofer is 4 ohms, that is irrelevant because it is driven by the 300 watt amplifier that is integral to the subwoofer.

Just connect the terminals of one of your speakers to the any of the input terminals or jacks of the subwoofer. That is all you need to do.