Willsenton R8 Tube Whining after swapping tubes

Hello, I recently swapped out all my tubes on my Willsenton R8. However I realized a whine coming from the three 6SN7 preamp tubes. The whining is coming from the tubes itself, not thru the speakers. The whining doesn't increase or decrease when turning up and down the volume. The replaceable tubes for the 6SN7 is stated in the usermanual. Replaceable tubes for the 6SN7 being: (6SN7GT, 6H8C, ECC32, ECC33, 6N8P, 5692). My dilemma is that I put in three 6SN7EH's tubes which are not stated to be replaceable tubes, and there was a very noticeable whine. Since then I've swapped out the 6SN7EH's for the original 6SN7 tubes that came with the amp. There still a whine, be it less noticable. Question 1.) Did I damage my amp in anyway? Q2.) Is it still safe to use the amp without causing damage? Q3.) Is there anyway to fix the whining? I'm also a 23 year old idiot trying to get into hifi, so dumb down the response if possible. Thank you lovely people and hope to hear back soon.