High level speaker amp out to RCA

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!!