Yes I found the McIntosh separates really un-engaging in comparison with the Diablo. I went into the at-home testing with quite a lot of respect and expectations of the McIntosh brand. But after the test I can't view McIntosh in the same light. Not really fair to McIntosh as I think my feeling is that there is nothing materially wrong with the brand, but rather the Diablo 300 is just punching way above its weight, in an integrated package which further decreases cost and increases value. I'm sure it can't touch far more expensive Gryphon separates, but I think the Diablo strives to be comparable at that level.
Having said this I don't have experience with the McIntosh mono blocks so I can't really comment, other than you would need to factor in the cost of all the upgraded interconnects and power cables, which are an absolute must (and will cost thousands for decent cables). For all non-believers do a blind test, and then comment. My non-audiophile wife can easily hear the differences in blind tests, even if she has a funny way of verbalizing what she hears. Also I just found the build quality of the McIntosh preamps to be a bit light and chintzy. It is light, it doesn't feel solid, and the screws on the chassis are not even flush mounted. Based on my experience with McIntosh brand, my totally subjective biased instinct would be to move away from McIntosh and not consider the mono's, but really that is not fair advice considering my lack of experience with the mono's. I'm biased based on my test and also because I'm totally satisfied with the Diablo.
You'll probably hate hearing this, but just like so many have said, I suggest taking a flight to Vancouver (quickly before Coronavirus takes over the country lol) and listening. The Gryphon dealer would be happy to arrange a head-to-head test between the Diablo 300 and the McIntosh mono blocks - give them advance warning and provide a Tidal playlist ahead of time, and they'll have it set up for you when you arrive. Fortunately I did not have to fly, but I was totally prepared to if needed (after saving additional funds). You can't possibly make the best decision for you without going to listen, so you can't look at the travel cost, which I know are high, as wasted money. And even then it's still not perfect as it's different speakers and different rooms, which messed with me a bit. There was definitely a bit of a leap of faith at the end, but you want to narrow that leap as much as possible. Don't know if you'll take me up on this, but if you do go, make sure to go multiple times on the trip with a break in between, even if on the same day. Traveling is hectic and psychologically there is pressure on extracting everything you can from your senses in a brief listening session, and that does not provide a very good quieting / leveling of your auditory perception with all of those factors going on. The first time I went I traveled a short distance, and arrived after meeting with family, in between events, and realized I was dehydrated. I left with completely wrong conclusions and at first decided I wasn't going to proceed. I went back another time, totally relaxed (and hydrated), and things came together and I had a much better assessment of what I was perceiving. I had to trust that the slight niggles I still had with the sound were more related to the speakers at the dealer, and once I got the Diablo home I was thankful that my gut was right. Sounded perfect to me even before burn-in.
Almost every retail item at this price point is negotiable, so yes I did negotiate. But you should follow your own path on the price points to negotiate; wouldn't be fair of me to disclose what the dealer let it go for. Just know that they have some margin baked into the retail cost and that is flexible to a point. Don't get greedy and just offer a fair offer that's a bit lower than asking. They will counter your offer of course, so expect that and it's up to you if you want to stick to your guns.
The Diablo 300 exceeds all of my expectations and I don't see how I'd possibly ever need to reevaluate. At some point when financially viable I need to find the equivalent for speakers - but that's a ways off. Currently my system sounds incredible, but I do recognize that performance is now limited by what my 803 d2's can do. Great speakers for me, but they are about a third of the value of the Diablo 300 so there are some inherent constraints in the detail the Diablo can push through them, etc. The Diablo signature sound is definitely present even when testing different speakers at the dealer - it sort of takes over, in a very good way.