States are a voice of their own.
You can't take it away or you might as well dissolve the state legislatures and rent out the governor's Mansion.
The first thing here is, are senators a voice for their state?
Probably not. They're a voice for their party. Generally senators will do what it takes to get reelected, which in the US is make lots of money to tell people how great they're doing, when all they're doing is making lots of money to tell them how they're doing rather than doing something.
Let's compare the EU to the USA. Both are unions of countries, however the EU is more a union of countries than the US, which is only one sovereign country.
The EU is basically run by the leaders of the member states.
The executive is split into two parts.
1) The European Council. This is made up of the heads of state of the individual countries, the president of the European Council which requires a double majority of the heads of state, the president of the European Commission who is nominated by the European Council which is the heads of state. So essentially totally controlled by the heads of state of the EU countries.
2) The European Commission, each commissioner comes from one of the 27 countries in the EU, and each commissioner is given a department. They are supposed to look after the interests of the EU and not their home country. The European Council nominates these people, the European Parliament (elected by the people) has to approve these people.
The legislature is split into the Council of the EU and the European Parliament
1) The Council of the EU. The presidency is rotated every 6 months among the leaders of the EU countries.
2) The European Parliament is elected by the people, with proportional representation within each country, that might be done regionally within those countries (like huge constituencies). But they don't get to propose laws, they just get to pass the laws that are given to them, essentially, by the heads of state for the EU countries.
Quite clearly they power in the EU is by the heads of state.
In the US senators are elected by the people and have no need to do what the state wants. In the past I guess the senators felt that need, but in the present day, it seems it's all about A) money, B) money and C) power of the political parties.
So, changing things to PR wouldn't really change anything in terms of state power.