You may want to read this. Or go directly to SCHEMATICS
When you read "5700 Ohms" in some circuit you found somewhere, that doesn't mean somebody calculated that within 0.1% and when you take 1% off, it begins to sound bad.
Reality is really totally another.
Rp is plate impedance, and Ra is transformer impedance.
Such a thing as "the" Rp of a tube does not exist.
Rp is the plate Impedance, specified at the voltage and current, used to specify Rp at.
Manufacturers have difficulties enough, with fools trying so simplify things down a silly level. So when they would provide Rp curves, that would only mess up the situation for those who do not understand it anyway. So they just give one operating point (OP), and one transformer impedance (Ra) , saying what output power and distortion results from it.
So Rp is determined by OP, and the next question is, what transformer Ra we must take. This is the trade off between Output Power and distortion. You just can't have it both optimized. Even so, what we call "tube sound" is nothing but "even harmonics" distortion for HiFi, and even+odd harmonics distortion for guitars.
For guitar amplifiers, we want them to sound clean at low power, and to fully distorted at high power, blowing off your ears. This is achieved by choosing Ra=Rp. In addition, loudspeakers are used with unusual high efficiency, which is achieved at the cost of additional distortion, and sound coloring.
In HiFi however, it's the opposite. Frequency range of recordings goes much deeper down than guitar strings, and we must take care, driving the bass chassis free of resonance effect (to avoid muddy bass). For this reason, we expect at the 8Ohms output, an 8Ohms speaker is connected, but we supply the speaker not with an 8Ohms signal. A much lower impedance is presented! In fact Some 3...7 times lower. So yes, the 8 Ohms speaker would be driven with 2 Ohms, if we take a factor 4. This results in cleaner bass, and a lot less overall distortion. All tueb amplifiers without feedback are like that. his is at the cost of Effiency, you just can't have both. A HiFi amplifier makes no sense if we say it has VERY much output power, but bad sound. So the choice is always, driving the speaker with much lower impedance as it's own. As said, the factor for this is 3...7 times. This choice is fully mandatory. So if your speakers are not loud enough already, you should not take a factor 7. Or, when they have excellent efficiency, but problems with good bass, like a back loaded horn for instance, you better take a factor 3.
But as so often, the numbers are only confusing for people who are only interested in the result and not what is behind it. This is why schematics have always ONE Ra written in it, whereas is reality this a a very variable number, and just a mandatory choice, somebody made at some moment.
So with the above in mind, please be not too critical with transformer impedance Ra.
If it writes 3k5 in the circuit, and you have a 3k8 transformer available, the maker of this circuit would have taken it just as well if he would have had it at the bench, and the schematic would have been published with 3k8. Though the impedance differs 10%, distortion and output power change only marginally. (Both will be lower). Even so, generally of you have an output power problem, take a little lower Rp as in the schematic, so you get a fraction higher output, at a fraction higher distortion.
Another reason not to blind stare yourself on exact Ra matching to some number, some person, wrote in some schematic is.... the speaker impedance. Speaker builders just write "8Ohms" on an 6Ohms or 11Ohms speaker, because if they would write anything else on it, you would not buy it. In reality, because of all the things written above, this is indeed not very critical.
To my opinion, any deviation of 10% is not critical at all.