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Exactly. When I speak fast I always say желанье, тобой etc. but when I want to emphasize something I mostly say желание, тобою... and so on.

I wouldn't say that it poetical or usual form [с тобой] and [с тобойу] - in spoken language the last vowel is reduced. The same thing with желание /желанье - и makes н soft and its reduced, IMHO.

Поезд мчится в чистом поле, чиста поезд, чиста в поле... :)