Effective recommendations to add warmth & smoothness while keeping bass control

Warmth and bass control are tightly coupled in most cases. Amplifiers that don’t properly drive speakers (woofers is where the challenge is) will sound brittle and thin. Match your speakers with high current class A or A/B amplifiers based on impedance, required power and the desired sound signature. 
Coda is a great choice but again depends on speakers. Keep in mind Coda amps are not warm sounding. They’re very clear and transparent. I’d say they are slightly on a cooler side sonically but with great bass control given proper speaker matching. Coda no.8 is offered in 3 versions and you can have Coda change your amp to whatever version you prefer out of available 3 choices. 
The no.8 does not run very hot. Much cooler than Pass. 
 

Thank you, @ghdprentice. 

I tend to agree that a tube preamp + solid-state amp will be the most effective approach. But since the Pass XA series runs in pure Class A, would that combination end up sounding too warm?

I don’t want to trouble you with specific preamp recommendations, but I’d like your general impression of Audio Research versus Conrad-Johnson. In general, are AR tube preamps more neutral than CJ? If so, would using an AR tube preamp with my current solid-state amp still be effective in adding warmth to the system?

@lanx0003 what speakers are you using? 
ARC tube preamps, especially models made in the last 8-10 years, are focused on adding as less of any type of coloration as possible. Some say they sound like solid state. I disagree. There’s smoothness associated with tubes but other than that they’re pretty much neutral. No bloating in the bass. Very accurate.
Great pairing with Pass amps because they don’t compound the effect of an already warm Pass sound signature.
If you want more of a classic tube sound with ARC then the older Ref1 or Ref3 would be a good fit. Huge bass, warm sound. 
As to CJ, they’re sweeter and not as neutral. I would pair CJ with Coda amps. 

@lanx0003 I am getting the CODA #11 today and I will put it side-by-side with my CODA #16 and do a comparison. The idea was to sell the #16 to raise a lot of cash to buy a DAC but I already raised all the cash I need by selling other gear that became redundant with the new DAC. The Imersiv D-1 DAC.

The #16 does not run hot, just warm. I am going to assume the same for the #11. The bass is the best I have had in the house with the Schiit Wotan in No Feedback Mode almost as good on the bass.  There is a great feature on the #11 on SoundStageUltra by Jeff Fritz. A reviewer I liked to read. That article gave me the idea of getting the #11.

After my comparison with the #16, I will compare the Schiit Wotan amp in my office with the #11. I now have 3 amps but only need 2 so 1 will be sold.

I paid $2457 including shipping for the #11. It has appreciated in price since COVID. There is a guy in SF that has been trying to sell the amp on and off for a while now (see eBay).

If you want to really take your sound up a notch. Take a look at the DAC in the ad I have here. It is the last photo. I bought the ugly DAC, and the pretty one is the $26k Allnic D-15000 Signature OTL/OCL DAC. It will give you both uber detailed and the most beautiful and smooth holographic sound. You need a great preamp with it though. I loved it with my Benchmark HPA4 (just sold) and also with the $33k Allnic preamp.

Schiit Audio Kara F (Sale Pending) For Sale - US Audio Mart

The DAC I purchased is a totally different sound from the Allnic. If I had the cash, I would get both. I am now going without a preamp since the DAC I purchased is worse off with a preamp (even the uber neutral HPA4).

If you have good amplification and speakers to start with that delivers the dynamics , applying DSP is the most versatile way to adjust the tonality in general, including adding “warmth”.  I do it using DSP capabilities baked  into Roon.