More for the debate, without providing an unequivocal answer:

Having gone, several years ago, from what was once one of Onkyo's better 'amplituners,' to a 950/770 combo, I would say I heard a marked improvement. I didn’t have your quandary though. I wasn’t considering a loudspeaker change because of tight spaces and I think my speakers, while not efficient, tend toward accuracy.

But, back to your question. I would answer: replace whichever is closer to “average.” If your speakers are a cut above the fray and your receiver is not, upgrade your amplification first. Even the built-in amplification in some very pricey all-in-ones often is found lacking in the opinion of many when listeners compared to separates. If your speakers are middle-of-the-road and the advances in materials and manufacturing technology enable better driver/cabinet combinations at prices that are not too steep for you, then upgrade the speakers first.

Finances may influence your order of upgrade as well - perhaps do the smaller cost upgrade first? If your amplification needs will be well met for $1000 to $1500, but the speakers you would really like cost $3000 to $6000, then try the amplification first. Who knows, your present speakers may 'come alive' when 'effortless' current and headroom uncompromised by the presence of high-frequency transients is available to those speakers.

And finally, if your plan includes upgrading both eventually, then relax a bit. The advantages of a good separate amplifier will not go away while you wait to upgrade your speakers, nor will the advantages of more accurate speakers go away while you wait to upgrade the amplification.

One more item to note: are you going to get 'combo' pricing from Outlaw if you buy an Outlaw amplifier relatively soon after buying your 990?

I hope it goes well for you!