High level speaker amp out to RCA

I have read many conflicting opinions on this topic, and the point you raise about introducing the amp distortion to the sub signal is very logical.bdp24 

I am having some issues with my RCA connection from my preamp. I hear the sub "interacting" with the speakers.  Sometimes it makes a clicking sound, other times there is a little click sound when I use the volume on the remote, up or down. Sometimes just straight interference sound, very rarely though.

My preamp is a Sunfire Vacuum Tube Controller, it has 2 parallel main out RCA style, and one XLR.  My amp, a Sunfire 300 Two, is connected with the XLR.  My sub/s, tried with single and dual, are connected using the parallel main out posts from the preamp.  The main idea was to go straight to the amp because apparently the Preamp is getting this interference. Just trying something new!!

Oops, the last sentence of my first paragraph above reads "It is his opinion (referring to Ryrhmik designer Brian Ding) that high-level hookup is preferable." I meant to say line-level (pre-amp), not high.

You would need a new buffer circuit or a voltage divider. Assuming the inputs are high impedance (25k or higher) I'd suggest you try 100K in series and then 1k to 5k in parallel. This would give you 1 to 5% of the amplifier output at the sub inputs. You could also use a pot instead of the small value resistor. 

Be careful as you certainly can provide too much voltage to the line in and damage it, so use a voltage meter at the small resistor to make sure you stay below 2V 

Seems like its not that straight forward.  I there an external adapter/converter that is just plug and play??

I recently purchased a new(used ) tube preamp. It does not come with extra set of 'outs' for a subwoofer connection. My subs are RCA level input only, so I am in the exact same predicament. 

I have a friend who designs some of the world's best loudspeakers who, taking pity on me, has agreed to put together the right package of resistors to make my speaker level connection into a line level with some spare speaker cable, some resistors in series and and soldered RCA plugs. 

Its not done yet but I can hit you back with the design and reports of how it works when I get them. 

.. More to come.