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

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 ...

I am not so sure I agree with the premise of the OP. entirely. No doubt the room is hugely important and intelligent use of treatments is paramount, but I dont hear room problems in terms of balance. Mids and highs do nothing to add to or detract from bass.