If the question was, "How do I get better overall sound out of my studio?" then the advice to add new monitors makes sense.
But,if the question is "What piece of gear do I buy to make better recordings?" then the best monitor in the world will not make a poor recording a better recording, it will only better reveal the performance limitations of upstream components in the audio chain.
If you already have a great mic, then spend your money on the the A-to-D and then maybe your mic pre. Only they can actually improve the quality of a recording, a set of monitors can only reveal what the playback system can do, it can't actually improve it.
With a new ADC/DAC you get improvements at both ends of the signal chain (recording and playback).
This is an audiophile perspective. It says spend a large percentage of your budget on the source, which you have already done with a quality microphone. Downstream components from the microphone cannot improve the signal that is already there, they can only distort it. The degree of that distortion depends on the quality of the component thru which the analog signal is passing.
Some of the posters here did not answer the question that was asked.
No doubt there will be some dissenters to this opinion, but it's the same in the audio world where many well-meaning people believe speakers to be the most important component in the audio chain. It is true that speakers have the potential to colour the sound more than any other component, but the truth is, the source component is always the most important.
Improve the source and you improve the whole system. When you have the dough to get better monitors then you will have better recordings to listen to.