You don't lack bass, you have too much treble

Being of advanced experience...   (I'm old OK)  a flat response being produced doesn't mean I'm experiencing a "balanced"  presentation.  Since I'm adjusting my listening experience to what sounds good to me a flat response as a baseline is where I would attempt to start but it's just that a starting point.  My biggest issue is the huge difference in recordings.  It's especially noticeable in the bass as it can go from "yeah that's it" to where did the bass go? to OMG that is way to much bottom end. 

The best I can hope for is most recordings sound pretty good without adjusting anything so a "flat response" is pretty much moot for actual listening.

To make a room sound less bright, are diffusers or absorbers, or both, suggested?  Thanks.  

Both. Absorption will reduce the brightness and diffusion will scatter the sound so room resonance frequencies are not emphasized. GIK has some nice slat diffusers in different absorption depths. 4" works well for most rooms. Most of their panels I've seen have both attributes. 

Or just get better components.

Better components ad infinitum will not beat a dedicated once for all acoustic room...

No speakers beat their room said some anonymus acoustician i forgot the name ...

It is incredible that so many people vouch for better gear design always, costlier upgrade, never for acoustics understanding and improvement perceived at most as secondary ...

Even miraculous Dr Choueiri filters can correct all stereo system which are all of them defective because of crosstalk, unbeknownst to most audiophiles,  but even dr. Choueiri need for the best optimized results a dedicated acoustic room and inner ears measures and HRTF tracking ... Psychoacoustics rule audio not mere gear design price...

Then asking for changing good speakers for costlier one is preposterous more than half the time and absolutely not comparable to acoustics improvement which can be and must be made ideally for the chosen speakers properties and type ...