It is quite possible that an incorrect tube replacement was made. Tubes are rated for gain with a parameter known as the amplification factor. There are tubes that are very similar but have different amplification factors. For example, the 12AX7 is a popular tube for low level amplifier applications such as audio preamplifiers. It has a relatively high amplification factor. The 12AT7 is similar but has a much lower amplification factor. Therefore, substituting a 12AT7 for a 12AX7 would result in a lower gain amplifier. You need to identify the correct tubes for your amplifier and then compare it with the actual installed tubes. Sometimes, there is a tube diagram inside the chassis. If not, you will need to get a parts list or a schematic.