On paper, the A5000 amplifiers are the same as those on the s950, , s750, s970, s770, s975, s775, and SX700.
In practice, there are crossovers (where the hi and low freq's ... cross over) and EQ's involved that change the sound perception dramatically: the PSR3000 had an amazingly balanced, full, warm, yet with punchy bass speaker set. (30w, like the A5000). then they lowered the wattage to 24w for s900, s910... quieter, less punchy bass, but still clean and warm tone.
Then came the s950, they'd heard the lack of bass complaints; and it was really, really punchy! Unfortunately, the treble was also overpowering, so that it caused me ear fatigue at high volumes (couple that with the onboard T4 drums being sampled in mono, along with any of the new brass/strings that were new going from the T2 chipset onward were also mono sampled). Then the s970/s975... still a 15+15 w system, but clear & punchy, without any high frequency ear fatigue (which is one reason I bought one).
The A5000 uses the less bass punchy speakers from the s750/s770/s775. So yes, if you output to an external speaker set, won't make much difference, but using the built in speakers for personal monitors, the sound... just doesn't convey the same energy without that deep bass component.
Try an sx900 next to an A5000 in a store yourself, you'll see.
But as I've stated; if the middle eastern sound palette and life control quarter tone scale is of no use to you, you're *much* better off getting the sx900, because that's what you're paying for from an A5000 over the sx600. (not even the sx700, which surprised me.)
Also, the previous A2000/A3000 also used the same amplifier set as their corresponding s7xx, but with a very boosted bass response. The speakers on the A5000 were... unexpected.
You can't add the strings from the sx900 per se, but there are sample packs out there (from Yamaha and others) that can give you at least a similar sound in most categories.
Under normal circumstances, the Genos 2 would've probably been teased by the end of this year, and the subsequent sx series would've arrived another 1.5 years after that. But with the AKM fire and parts supply shortages, these are far from normal circumstances.
If you're waiting for the next sx series? I can tell you not to expect the next full upgrade until well after the next Genos. (there *have* been half step upgrades, like the PSR2100 1 year after the PSR2000, and the s975 2.5 years after the s970, and the CVP210 1 year after the 209). since this is merely a function update, yes it's possible we'll see a mk2 of the sx900/sx700. Keep in mind these half step upgrades share the same chipset and architecture as their originals, just with a couple of minor additions (in the case of the CVP210, not that minor: smoother piano sample and USB B port!)