Hi Yani
I haven't seen the RX1 in the flesh but from the pictures I've seen it does look to be a slightly larger cabinet. Apparently they use a new bass/mid driver and have thicker cabinets than the RS1. They retain the reversed river arrangement with the bass/mid driver above the tweeter (ala Mission 760/780 models from the early 90s) but Rega have also gone to a circular grille cover for the bass/mid but semingly no grille for the tweeter. The latter would put me off as I like to keep my drive units fully protected.
I was aware of the imminent launch of the RX1 and have been since WHFS&V's reports from the Bristol Show. Given the pricing of the new models I wonder whether Rega will continue with the RS range alongside as the more affordable alternative. The RS1 is known for its clarity, refinement, and a great sense of timing and when I've heard them in the past I've enjoyed them. I'm not a fan of lots of bass and would much rather be able to hear right into the crucial mid-range of a recording, but also get a sense of the timbre of low strings etc without that being obscured. I'm confident the RS1 will give me everything I want, essentially being a modern take on the classic small monitor approach which I like. They probably wouldn't suit you, though I think it would be worth you giving the new RX1s a try to see how different they might be.
Your Celestions are no slouch. Sadly the name has disappeared from hi-fi speakers these days, though it is under the same foreign ownership as KEF if I remember rightly. They made some great speakers and for the level of depth and sense of scale you are talking about, a 7" bass/mid driver is going to be difficult to match with something smaller. You might well find the Monitor Audio Silver 2 worth trying with it's 8" bass/mid driver, though I suspect any 500 pound-ish speaker is going to have a fight on its hands to supplant your Celestions. The larger MA stand mounts usually require plenty of space to ensure they don't lose clarity, though this latest range is much less prone to muddling than the previous RX2 so it might just suit your needs as long as you feel there is a genuine step forward from the A1s.
The other speakers worthy of your attention are the Dynaudio DM2/7s. Dyns have a weighty, substantial sound (ultimately a bit too weighty for my tastes, and I prefer my DM2/6s with the port bungs in place, even 50cm from the rear wall) and the 2/7s are a decent sized speaker with very high quality drive units.
Otherwise, if you want a real balance between a sense of scale and the rhythmic and spatial qualities of smaller stand mount monitors it might be time for you to up the ante and look into the PMC range. DB1i isn't made any more but you might be able to pick up a nice 2nd hand pair, and these are beautifully balanced speakers and the transmission line design really helps with the sense of scale. Alternatively there are the current models the Twenty:21 and Twenty:22, though these will shift your budget much more skywards compared to the other speakers we have discussed.
Keep us updated with how you get on, and I'll report back on the RS1s once they've arrived and I've spent some time with them.