High level speaker amp out to RCA

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. 

Is there an external adapter/converter that is just plug and play??

It is a voltage divider that erik_squires referring to:

https://www.crutchfield.com/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Adapter.html?tp=...
attenuation: -30 dB, should be safe to connect to your subwoofer.

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.

Thank you, greatly appreciated!!

It is a voltage divider that erik_squires referring to:

https://www.crutchfield.com/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Adapter.html?tp=...
attenuation: -30 dB, should be safe to connect to your subwoofer.

Thanks, will try this out and see how it goes!!

Thank you all for your help!!