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)

I found a passage in the Audio Note Kits manual that states that 'for 15 seconds after power on the amplifier will hum as DC current stabilizes but it should disappear completely'.  With ordinary WE style 300Bs this translates into a low level hum from the amplifier which is transmitted into the loudspeakers.  I'm guessing the 300B XLS is more powerful and apparently more sensitive to this transient DC ripple, thus the loud howl transmitted through the loudspeakers. KR website states that this tube has 4 filaments and 32 'functional cathodes'. Perhaps the construction of this tube makes it ultra sensitive for those brief 15 seconds. (and it does stop completely)

Also the Texas Instruments  LM-1084 5V DC voltage regulators in these amplifiers  are rated at up to 5A, with 300BXLS filament draw of 1.9A.  it is a parallel single ended monoblock but there is a separate voltage regulator for each tube.  I guess it sounds safe enough to go with the better sounding KR 300BXLS as long as the 15 second loud howling/DC settling noise doesn't damage my loudspeakers.

My Audio Note Kits Interstage Monoblocks do not have soft start circuitry, which according to Roger Madjeski, would prevent the brief loud noise on power up  from the EML or KR 300BXLS in my ANK amplifiers. 
Is it common for low powered SETs or even low to medium power like mine (20 wpc) to NOT have soft start power on? What are the pros and cons of soft start circuits?
Thanks to all for your helpful comments!

It looks like this conversation just fell off.
Does anybody have any general comments, raves or problems with the basic 300b interstage monos, considering a pair.
Hi Tubesrule:
Just noticed your comment; been a while since this conversation saw activity. Anyway, as it turns out, problem described above related only to gassy 300b tubes; the ANK 300b Interstage are great amps, both the PSE (20 wpc) and SE (8-10 wpc) versions.  Just a couple of months after buying the ANK PSE, I went to a VAC Signature 200 IQ for more power.  However, the ANK are especially wonderful with Voxativ; I’m sure other high sensitivity loudspeakers as well. I’ve got like new (2 months use) ANK PSE Interstage Monoblocks if you’re interested with optional transformer coupled balanced inputs. Good luck!