Audionotekit Kits Interstage 300B Monoblocks and KR 300B XLS

I recently purchased a pair of wonderful Audio Note Kits (ANK) Interstage Parallel Single Ended Monoblocks, factory built.  They arrived with Shuguang 300B's which sound quite good.  However, I have 2 pairs of unused KR 300B XLS Power tubes I bought 10 years ago. I read that they are drop in replacements for a WE style 300B but will only deliver standard 8 watts, which is fine.  I installed them in my new ANK PSE Interstage Monoblocks and they sound incredibly good.  The ONLY thing that concerns me is, at turn on, when power is first supplied to the monoblocks, there is a loud howling/moaning sound from each loudspeaker lasting 8-10 seconds then completely disappearing. (Sounds like a toilet with a bad inflow valve)  I don't think the loud howl is doing any damage to my KEF Referece 3 drivers, but I am a bit concerned as to the safety of my loudspeakers and new amplifiers.  From what I read the 300B XLS has higher filament and plate currents and plate voltage but it shouldn't stress normal 300B amplifiers, which will provide the standard 1.2 A to its filament.  Thank you for any and all advice!  Again the 300B XLS tubes sound fabulous in the ANK PSE Interstage Monoblocks after the initial inrush of power. (10 seconds or so)