Small or large sub for music

Since you have both why not just try them both and use the one you like the best?

There are no speakers currently manufactured that " need " subs. If a person wants to add a little more foundation to the music they may choose to add a sub or two or one might suppose a swarm of subs. 

I'm using a single small REL t5si sub with my Harbeth P3esr speakers.  I moved it around the floor until I got a good sound.  Running the speakers full range and I keep the crossover on the sub set fairly low.  The sub volume is also very low.  It's a subtle difference, but the arrangement creates a bit more body in the bass.  The speakers are actually fine without the sub.

I use a different system for Home Theater, with a noisier subwoofer.

Remember that good subs don't simply add bass reinforcement, but contribute to the overall listening experience by adding life to the mix. Live concert halls and even studio mixed recordings have lower frequencies adding an ambient and more lifelike "feel" to pretty much everything, and although your speakers might not "need" this, I know I need it for things to sound like real music. My current Heresy IIIs are great at producing realistic sounding bass within their range, but really drop off at around 58hz or so...my subs take up the rest and the resulting overall low frequency coherency and "room charging" gets the mojo right.

After setting the low-pass filter, I use pink noise to set the levels of all my speakers, including the subs, to 75 dB SPL, then listen to a variety of recordings.  My goal is to not be aware of the subs.  A Velodyne SMS-1 bass manager sets crossover, slope, and acoustic room correction for the subs.

As an amendment to my OP, I did buy a pair of KEF Reference 1s.  Hope I don't regret it, given the LS50s supplemented by the HGS-10s sound so good even for large orchestrations.

db