Need advice on adding subs

I’m looking for advice on adding subs to my system.

I listen primarily to small combo jazz and classical but I’m disappointed by the classical symphonic reproduction. I can only enjoy symphonic music on my headphones. Perhaps, I could improve symphonic on my main system by adding subs.

I don’t listen at high volume nor am I a bass fanatic. I just want the better sound. This is strictly 2 channel system. I have no interest in HT. I live in an apartment.

My system:

Balanced Power Technology BP-1 Conditioner
Toshiba lap top with JRiver feeding via crossover Ethernet to
Sonore Rendu
Wyred4Sound Dac-2
Warpspeed Optical Attenuator
First Watt J2 Output power 8 ohms30 watts 4 ohms15 watts

Reference DeCapo MM 2-way Monitors

The speakers are a simple 1st order high pass passive 3kHz x-over to the tweeter. I believe the woofer is designed to rolloff naturally at both ends of its band width.

I’m open to any ideas. Lets say up to $2000 budget.