Pro ject tube box s

Anyone have a good tube upgrade for this little unit.Cheers Bob
Well I ended up buying a matched pair of Mullard tubes from tonal tubes.ive left the tubes break in for the last 3 days along with a morrow ph3.I have one more ph3 on order that I’m waiting for.
Today I’ve had the time to set and listen to my system all day and I’ve notice it’s finally starting to come back to life.First hours were flat lacking bass and highs were not where they should be.I was thinking well that was a waste but I’ll keep going
That was around 3 I’ve been listen off and on now hour or two take a break etc,now it’s 12 and the system has changed.
Detail is great ,bass is tight ,and highs are back towhere they should be .Tube box is doing its job and soare the tubes.I guess it’s just a audiophile thang Cheers
Can you use ECC81 (12AT7) instead of ECC83 (12AX7) in this circuit ? 

I've been searching for the best NOS ecc81 (12at7) for my ex amp and discovered some amazing vintage tubes from telefunken, sylvalia, matsushita