I have concerns about their use in enclosed tube amps like the SP14 however, in regards to heat. In my research I have found that a typical preamp enclosure with 4-5 tubes plus a couple of transformers could be operating at 50-60c (122-140F)
One user found that after less than 200 hours his Mundorf SIO's had dropped their rated value. In another case the SIO's failed completely.
On the plus side others have reported no problems even after 4 years of service.
One of the previously mentioned user's whose SIO's had failed was told by a Mundorf rep that at high operating temperatures the oil degrades the silver foil causing them to loose rated value.
Up until 2011 the Mundorf website was specifying an ambient operating maximum temperature of 70C/158F but currently has downrated that to 55C/131F for their Silver in oils.