Your Focusrite Saffire (if its the vertical model) has 8 balanced outputs, s/pdif (2), and headphone jacks. Since you are familiar with Focusrite, their Scarlett 18i20 model offers a complimentary output count to your Saffire.
If you'd like to incorporate the s/pdif port on these interfaces, you could use a converter like this one:
https://www.amazon.com/dp/B00F4ORTZ6?ref=emc_b_5_i
This would be fine for a subwoofer since most common subs offer an RCA port for connectivity.
Note that this model (18i20) the monitor out connections are shared with the headphones jacks. Other interfaces allow you to have separate channels between the monitor out and head phone jacks. Not the case here which is why the specs say a total of 20 (independent) channels. To get to 20 you would use the digital out connection to an ADAT unit for an additional 8, like this one:
https://www.sweetwater.com/store/detail/ADA8200
Note that the output ports on this Behringer ADAT are XLR vs 1/4" phono.
Thus:
8 - channels, 1/4" TRS connections
2 - channels, s/pdif
2 - channels, monitor out which is shared with headphone jacks 1 & 2
8 - channels, via digital out connected to an ADAT
Total = 20
An audio interface for HW, we are mainly (if only) concerned about the D/A quality and number of outputs. The MOTU 24Ao model offers only output connections. A possible negative on this unit is its output connectivity. You have to use Phoenix connectors or a DB-25 snake. MOTU also has a 16A model that has 16 standard 1/4" TRS connections for output, but also16 for input too and is more expensive than the 24Ao!
Hope this helps,
Danny B.