replace Mcintosh with Luxman

I’m going to assume you meant the Luxman M900U power amp, not the C900U preamp. Having said that, you know that the M900U is Luxman’s best SS amp, and it has similar rated specs to Mac’s 611, other than the M900U is even heavier than the 611 (by about 4 Kg).🏋️

I’ve not listened to either, and short of actually listening to one, you’re going to have to take a big leap of faith if you decide on the Luxman. Now, I will admit a bias towards McIntosh as I use McIntosh amplification in my own system. But Luxman makes superb products, and they continue to offer both tube and SS products just like McIntosh does.📻

The only technical comment I will make is that the 611 was purpose designed and is purpose built as a mono amp, while the M900U was designed as a stereo amp, so when you are using the M900U as a mono amp, it is being used in a way slightly different from the designer’s intent. But this is likely irrelevant given the quality of design and construction of Luxman amps.📰

So what do you do? An analogy would be you having to choose between two non-identical twins in your family. They both have pluses and minuses, and if you were told you could only keep one, well that’s an impossible task that no parent would ever make. In a family situation, you would choose to keep both, no matter what.👬

But these amps are not emotionally close to you like your kids would be, so you do have the option of choosing one over the other.

If it were me, I would gather more information and take the following steps:

(1) You’ve made a significant financial investment in your speakers, and I think Magico should be willing to chat with you, at length, about the merits of these two amps. If they tell you they’ve never listened to either, well, they CAN tell you what amp (or amps) were used in the design and voicing of your speakers. This knowledge might be enough to suggest one of your two choices might more closely resemble the amp (or amps) used by Magico.🎷

(2) You should search for a dealer who actually has both amps in stock. I don’t know your financial circumstances, but if I were considering dropping 40 large on a pair of amps, I would also budget for a lower-48 road trip so I could actually listen to them side-by-side. I’m going to assume there is at least one dealer in the US that stocks both amps. If you can do this, you might well hear something that would push you to either buy the Luxman’s or keep the Mac’s. Of course it would be even better if you could find a dealer stocking both amps AND Magico speakers!🏨

(3) If (2) above is not an option, I would double down on (1) above AND I would begin contacting Magico dealers across the country to solicit their input AND find out what amps their customers are using to drive their Magico’s.👍

I wish you well on your quest. Please let us know what you decide to do, and what you learned during your journey.😎

The m900u mono was exceptional when I demoed it and way better than the 509x which I had demoed first that same day in the same store (TAD ME-1 speakers). Though, I am not sure if the Luxman in stereo has enough power for the Magico S5 V1 for a big room.

I have recently bought the CODA #8  ($6.2K) and it sounds a lot like that Luxman m900u. I drives my hard to drive Thiel CS3.7 easily and likely your Magic S5 V1. However, since you are willing to go to $15K MSRP you should consider the CODA #16 which is the higher model from the #8. The CODA is American made and likely offering you more value for the same price vs the imported Luxman. 

Not a fan of MAC's.

My Magico S3MK2 are powered by 2 Luxman M900u amp (using C900u pre-amp)
without knowing your room i can almost assure you you will get improvement in all aspect 9more refined sound, more dynamic and micro dynamic, plenty more details and control over the speakers)
a friend of me have the same combination (Luxman 900 series amp & pre + Magico S3MK2) but also Mackintosh MA 9000 he use to power his focal speakers on the other room.
one day we switched the amp just out of curiosity, so we powerd his Magico's with the Macintosh, the Luxman combination was far better to our ears

I have a better product for less if you don’t mind having to see the meters , the Excellent Coda CSIB integrated amplifier stereo timesgreat review ,best of 2020
for $6500. The best integrated under$10 k 
and better then any separates made in USA  10 year warranty
and powerhouse 3,000 va potted, not open transformer 
Mc ,or lux man 1/4 the size and 3 choices for power 
the lowest 150,300,600 has 18 first watts in pure class A.

Hello,
I would call both manufacturers and ask them if the other brand works well with their product. Also, talk to a dealer who is a dealer for both of those products. They might say it works great or give you a better option. Make sure you let all the people know as much information as possible. Type of music you like. Size of room
Do you have subwoofers and how are they connected. 
Treated or untreated
The cables that will be connected 
All of this information makes a big difference 
I hope this helps.