SS Amp Upgrade Thoughts (from Simaudio) with Magicos

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.

I suspect your existing amp is enough, but you need to KNOW if that is true.

the question is: these speakers, this location, at my listening level, for content with dynamics, Will I benefit from more power?

btw, maximum component depth is _____?

I suggest you find something twice as powerful as existing that you can return, buy/listen/answer the question.

once you know the answer, keep/return what you used to get the answer

btw, I chose Furman Power Conditioner P-1800 PF R because it's capacitors store/deliver 45 amp peaks for the 'power in' side of those dynamic needs.

https://furmanpower.com/products/p-1800-pfr

I paid far less when an open box popped up for sale years ago

Before changing any equipment I suggest you try using some EQ adjustments.  You want more fullness and slam, but you listen at low volume.  The Fletcher/Munson curves show how this could be heard as a lack of fullness.  Older equipment used to have a "loudness control" to counteract this.  EQ can simulate a loudness control and it may solve your problems without changing equipment.