Speaker recommendation: floor standing or shelf, 100% music, large room

You could find a used Magnepan MMG for $400. A pair of those would be fun in a large room and I think you'd be pleased with what they'd do to your preferred music.

You might be able to turn up a pair of old ESS AMT speakers, too. The low end isn't perfect but you can replace the woofers and perform other upgrades. The AMTs are sensational, though. I found a pair for $300.

I'll second Magnepan for a larger space. I've only heard the 1.7 and 3.7 though, so I can't vouch for the MMG, but the others are great. Out of budget, but a set of 1.7 with a sealed REL sub sounds amazing. The 1.7 (definitely going to have more bass than an MMG just due to size of the woofer) sounds really good without a sub though, so the MMG still might be enough for you.

American speakers are designed to be used in large living rooms. Boston Acoustic speakers for example are tweaked for an 'european sound' before exported to Europe where the architecture is supposedly different. Which doesn't make a lot of sence to me but looking for american speakers should be an good idea.

Those ER-4P have an exellent frequency response which won't be achieved with 400$ speakers. Speakers with a slight high freq roll off might be preferable according to the ER-4P curve though large room acoustics generally profide for such an characteristic. I think you will have to sarcrifice some low freq and buy speakers with a flat mid curve which probably is best achieved with bookshelves. Floorstanders would be more preferable however because they generally have more bass due larger cabin volume and therefore a "larger sound" which of course is nice in a large room. Floorstanders have a habit to give a more coloured sound due the large surface of the cabin, therefore is the weight of the speaker of  importance because heavy speakers wil vibrate much less than the light ones.

In my experience low bass music craves for speakers with an solid bass response because with bass shy speakers the idea of frequency loss is very fast obtained. My speakers don't have a lot of bass for instance which doesn't bother me listening to Hip Hop but while listening to Pink Floyd or Led Zeppelin it's very noticable. A subwoofer is indeed not decirable because you'll get phase interference with your speakers.

I can't help you chosing the exact speaker because I live in Holland and I only know about European speakers but I know the Totem Sttaff's are splendid. I would look for a used for a pair of Totem Mites en buy the relative cheap Naim Nac A5 cables to get very soft highs, incredible mids and a better low foundation. Though the NAD is possibly is a bit flat sounding so silver coated copper might also be a good option to give a bit of extra spark. Though that also could be achieved with the right interlink. You might not be a 'cable believer' but it does make difference IMHO so you might want to keep that in mind. Also powercables can do a lot of good which I found extremely hard to believe but it does. Though in a low-end system it's probably not a worthwhile investment.

In my experience 800$ speakers sound at leats twice as good as 400$ speakers so you might want to keep that in mind when it's an option.

Hope it helps