SourcePoint 888 owners – low-end bass in open rooms?

Some recordings sound excellent and punchy, but many leave me wanting more low-end fullness and physicality.

I agree with Tablejocky, your source is the issue. 

Room issues are usually nodes and nulls. A larger room will be better for bass than a smaller room, but it will take more power to fill it, but again you have satisfactory results with some recordings so room does not seem to be an issue at the MLP.

Subs probably will not help unless you have a way to boost the bass signal on the recording, aka eq.

If you add subs you will also need to add an external active crossover because if you don't you will be matching the subs output with the output of the 888's at their weakest point so the output of the sub will not be any louder that the output of the 888's or else you will have a mismatch in the low end output between the two speakers.

Given the "things I like" don't mess with those aspects, a sub or pair is the way to go. Re" "external crossover", nonsense. Most good subs have that built in; check out REL; you can start with one to see if it makes a difference (it should), and place it according to REL's recommendations. Your description of room volume is deceptive, as you have an open wall; your actual dimensions need to include the adjacent space, and it's no surprise that your 88s don't produce adequate bass energy.

FWIW, heard these at the Montreal Audiofest last year and marveled at the low end.

I have 888’s in a 1300 sq ft room. They are shared with my home theater, so I have 3 subs. I do measure with REW. I drive the 888’s with a McIntosh MC312, preamp is Anthem AVM 90. The subs are time aligned with a miniDSP 2x4 HD. The Anthem has ARC Genesis for room EQ. Crossover point is 80hz. This is probably too much information, and doesn’t help your question? But, in my room the 888’s integrate well with the subs. Some songs still sound light on bass, it’s just the songs. I do have a McIntosh MQ112 (10 band graphic EQ) for individual song material. Since some songs sound okay to you, you may want to try a high quality graphic EQ first for those few songs that are heavy or light on bass, and treble. I find my older rock and roll songs need help, and they are all different. Like previous comment, the 888’s are typically not light on bass, especially for music only.

The non-intuitive solution to this is to tame the mid/treble.  Overly bright/untreated rooms keep too much high frequency energy and can make the speakers sound unbalanced.  By increasing damping the bass is allowed to emerge like an old ship at low tide.  
 

You can experiment with this free by using curtains and blankets.