Just my personal opinion but if you're looking to buy new I would not swap your Monitor Audios for either of the speakers you mention. The debatable performance increase would not translate at all well in money spent. I think were you to do it blind you'd be contributing to "Your most dissapointing upgrade" thread rather quickly. I'd say the RS6 are on par with Tannoys granted different speakers in many ways. And comparing to standmount ATCs to floorstanders isn't quite fair to either.
If a question would be can you do better for circa the money you're looking to spend, then the answer is a resoudning yes as long as you buy second hand. Buying new I think you'd need to double the budget to have a worthy sound quality improvement.
That's all as long as you're relatively happy with what you currently have. If you're not than changing speakers is usually the right move.