...no such thing as flat speakers
True fact. Even if you spend $100k on speakers, they will exhibit spectral anomalies as well as intermodulation distortion, ringing and heat-related changes. Just less of them.
Another true fact: it is entirely possible to make great records with mediocre speakers. It just takes room treatment, proper placement, referencing alternate speakers, and - most important - ear training.
If you spend enough time listening to a pair of speakers, assuming the room isn't deceiving you
too badly, eventually your ears will come to "know" what a good record sounds like on them. All you have to do then is match that sound and you're home free.
Of course, there will be some frequencies that cheap speakers just can't reproduce, especially at the extreme low end. No problem. Start by filtering most of those frequencies out. You don't need them. Then use a visual aid to see if the lows you left in are too much or too little.