I looked at this for a bit last night, and ended up sort of on the fence about it. In spite of that broad plateau in the response 600 Hz to 1.2 kHz, the response stays within about +/- 2 dB. That's better than an awful lot of speakers, however it will still color the sound slightly, and the emphasis is in a frequency range that would annoy me. If it were a broad -2 dB dip in the same range, it would not bother me.
If I were wanting to build one of the Seas kits and not spend a whole lot, I would given the Idunn consideration. It uses the DXT tweeter, which is the same tweeter used in a number of highly-regarded speakers and is what ctrl and rick are using in the Directiva they are building. ctrl might have some advice to share on how to get more from the tweeter. The woofer used in the Idunn also seems to be much better behaved vs. the one in the Bifrost. Another possible advantage of the Idunn is that you might be able to find a suitable enclosure read made or knock down. I know that this was once true, but I'm not certain whether this is still true. But if it happens still to be true that you can find a suitable enclosure off the shelf, then it should be easy to assemble and the total cost for a pair (buying parts including crossovers from Madisound) will be about $550 + cost of two enclosures. It could make for a nice pair of speakers for not a whole lot of money.
If I were willing to spend more and do something a little more ambitious, I would consider the KingRoy mk III. My only reservation about it is that whereas it is a sealed enclosure speaker, the driver is excellent for ported subwoofer application and not ideal for sealed enclosure application. Were I to build this speaker, I would either go the route of the taller version using dual passive radiator as they suggest, or else make it a ported speaker, with a larger enclosure as needed for a proper ported design using that driver. This is covered adequately somewhere in suggestions and plans for a building a subwoofer using this driver. This would surely be an amazingly good speaker, although not a budget-friendly speaker.