Hello and thank you for your question.
Using a fuse that is rated much lower than the value specified for the device will almost certainly cause the new fuse to blow right away. The same is true if you replace a fuse marked "T2.5" with one that is just marked "2.5". The "T" stands for time delay. It means that the fuse will not blow unless the current passing through it stays high for certain amount of time. Many amplifiers use these types of fuses because they tend to pull a brief surge of current, that can be higher than the fuse rating, when they are first turned on. A regular "Fast Blow" fuse would blow immediately, but the Time Delay (or slow blow) fuse tolerates this short surge.
Now that does not mean the lower rating fuse is the only reason the fuse blew. When a properly rated fuse blows it can be due to one of two things. A temporary external surge, or a fault inside the device that is causing it to pull too much current through the fuse. The only way to tell, short of having it tested by a technician, is to replace the fuse with another properly rated one and see if it blows again. If it does, then you need to get your amplifier repaired. Never use a higher rated fuse than what is specified or you can end up causing more damage to the device.
I hope that answers your questions. Let me know if you need any more information on this issue.