Apart from the two on deck 17, where you may get some late night noise from folk walking from the Crow's Nest to the lifts, I don't think there's a poor location amongst the PF cabins.
There are a few on deck 8 above the theatre but, unless you like to be tucked up in bed with your Ovaltine by 22:00, they won't be a problem. The gym and thermal suite won't be open at sparrow's fart or late at night, so the ones under them on deck 4 should be fine also.
We, like you, are treating this as an 'extra' so went for the cheapest option. We though booked when the cruise was first released; that was out of necessity because we needed an accessible cabin. Luckily, the select fare at the time was the same as the saver fare, with the added benefit of the OBC, for which we received a recent boost of an extra £200.
I note the current select fare in a PF cabin is the same as we paid, but with an extra £100 OBC. That said, the lowest grade accessible cabin on the select fare that was available a couple of weeks ago is one of the promenade fronting balcony cabins on deck 8, and at £1500 more than we paid, I would not have thanked you for it.
If we could have guaranteed an accessible PF cabin being available this late, we would have done as you have, and saved a few quid. You indeed have got a bargain!👍