I just wanna check was my little mark 250, older version, ever been opened or changed in any way. I was at the guy that opened it and told me that because of the input power of transformer that is about 53,6 of something it can not have more than 150w even on 4ohm and it was suspicious that on transformer where white marks of a glue on place where transformer meets electric cables. ( pardon my english )

Transformers are impregnated with an insulating varnish... some of that often bleeds out... no worry.
Do NOT beleive any advertising regarding POWER capabilties of amps... they are OFTEN just marketing values. The transformer voltage is NOT the way to rate the power of the unit.The actual rating on the amp is 150 watts RMS into 8 ohms, or 250 watts into 4 ohm load. The "guy" doesn't know what he is talking about... find a more competent tech next time. A 53 volt transformer can generate about 46 volts plus and minus... A linear type amp can swing probably 50 volts peak with this or around 33 volts RMS AC output... at 4 ohms this will

When using amps,, you can't go by the number of the amp to determine what wattage the unit puts out, after checking on this amp,, the white marks that your seeing on the cable, is probably just some of the thermal paste that you apply to any heatsink or CPU ,, nothing to worry about, now as far as the wattage,, here is what they say about that amp, so there is a difference between the 4 and 8 ohm set ups
The Markbass Little Mark 250 bass amp head is a high-quality bass amplifier with a solid-state preamp and an analog power amp. The Little Mark 250 delivers 250W of power into a 4 ohm cabinet, or 150W into an 8 ohm cabinet. This Little Mark has a 4-band EQ, Markbass' VLE (Vintage Loudspeaker Emulator) and VPF (Variable Pre-shape Filter) filters, as well as both Gain and Master volume controls. The front panel accepts both a passive or active bass 1/4" input, as well as a balanced XLR input for acoustic bass pickup systems or a microphone. On the rear panel you'll find a Neutrik Speakon combo jack output that accepts both Speakon or 1/4" speaker cable as well as a second 1/4" speaker out jack. You'll also find a balanced XLR out, unbalanced tuner out so you can tune as you play without passing your signal through pedals, and a Send and Return Effect loop which saves you from losing your signal if one of your pedals dies or a cable in the chain gives out.