Looks like I've found why original 37xx bootloader doesn't work with 34xx firmware (Alviora, thanks for helping me with tests!).
I believe it's because 34xx firmware is too big, so the 37xx bootloader crashes while reading it. This also explains why some older 34xx firmware versions work fine with 37xx bootloader.
This issue could be fixed by compressing 34xx firmware and adding a special loader which would decompress it when executed. In a few weeks I will probably finish that piece of software and post here something for testing.