SS Amp Upgrade Thoughts (from Simaudio) with Magicos

Well, you’ve got a bit of a tough combo here.  Your speakers are a sealed design that offer quick, tight bass, which is probably what you’re hearing and to get more you likely need a bigger, beefier amp that may exceed your depth limitations.  I would’ve recommended something like a used Plinius SA103 Class A amp, but alas it’s 18” deep so likely won’t work.  Another potential option would be to buy this McCormack DNA-1 and have it sent directly to SMcAudio for full upgrades and for under $4k total you’d have one helluva amp, and these amps tend to have a fuller, meatier sounding bass and mids that may work well for what you’re looking for and at 15” deep should fit in your cabinet.  Just one option to ponder FWIW, and best of luck.

https://tmraudio.com/components/power-amplifiers/mccormack-audio-power-drive-dna-1-power-amplifier/

I personally like Simaudio equipment a lot. Have you heard any of the First Watt SIT amplifiers? They have a very high slew rate which translates into very fast transient response. I would think that a new SIT-5 model would have more than enough power for your listening level. They will deliver enough current to make those Magico's really sing.  If you have enough real estate for a pair of these amplifiers, you should give them a try. You might be able to find them used if you are patient. Very quiet and will not give you any listening fatigue, ever!

Coda No.8 would be a good fit. It can be configured as 3 different versions. Even the V1 with most Class A power should be a good match. Coda is a very transparent and clear sounding amp with excellent resolution. Very fast and articulate bass. Not particularly warm which is why I think it will give you what you’re looking for. 
Boulder 866 is an excellent integrated amp. Its sound signature is more refined than the Coda. You will get enough power with the 866 and it sounds great at low volume matched with the right speakers. Purely from the performance standpoint I’m pretty confident it’s a nice improvement over your simaudio combo. You don’t need a separate streamer fir the 866 with a built in DAC. Get a Roon subscription and set up a computer with roon core or pick up a Roon Nucleus One. You’re all set. 

Another option is to go up the Simaudio line. But you would need to replace both the 330a (I disagree with it being the weakest link..it’s a nice amp may be not a perfect match for a current hungry A3 but I think it’s on the border). 

I have owned the Benchmark AHB2 and Coda # 8. I took the AHB2 to a dealer and hooked it up to the Magico A3. Terrible, not enough power.

The CODA #8 will work well as would the cheaper Schiit Wotan. However, let me suggest an amp that I like better than the #8 and the Wotan. It is a 20-year-old CODA #11. I see an expensive one on eBay. I paid $2457 with shipping last month for mine.

With my new Imsersiv D-1 DAC, which is a firehose of sound, the #11 was preferred a tiny bit more than the CODA #16 amp on my Livingroom Yamaha NS5000 speakers.

I moved my #16 amp to the office where I have a used $1000 Magnepan Mini speaker. I cannot believe how good the #16 is on the Maggies. Again, I use the Imersiv D-1 DAC on the office. A crazy price differential with the DAC and amp compared to the speaker but there is a ton of magic happening.

The #11 and #16 are my end game amps and both would be great with the Magico A3. 

I agree with the Coda #8 solution. I had this amp and it is very good for low volume listening.

I don’t have experience with the 390, but I have made significant gains in my low volume performance by improving the digital chain. Something to consider for the future.